Cleanup __fish_complete_subcommand

The external-commands-only completion was briefly added in 3.1.0 and removed
in 3.1.1 (see #6798), which means we can remove some dead code.

Maybe we should just remove __fish_complete_external_command - it could break
users, but then again, we don't really have a way to stop people from starting
to use this deprecated function. The underscores ought to communicate that
this is function is private to fish but that is not enforced.

share/completions/and.fish

@@ -1,3 +1,2 @@
 complete -c and -s h -l help -d 'Display help and exit'
-complete -c and -xa '(__fish_complete_subcommand --allow-functions-and-builtins)'
-
+complete -c and -xa '(__fish_complete_subcommand)'

share/completions/begin.fish

@@ -1,2 +1,2 @@
 complete -c begin -s h -l help -d 'Display help and exit'
-complete -c begin -xa '(__fish_complete_subcommand --allow-functions-and-builtins)'
+complete -c begin -xa '(__fish_complete_subcommand)'

share/completions/if.fish

@@ -1,2 +1,2 @@
 complete -c if -s h -l help -d 'Display help and exit'
-complete -c if -xa '(__fish_complete_subcommand --allow-functions-and-builtins)'
+complete -c if -xa '(__fish_complete_subcommand)'

share/completions/not.fish

@@ -1,2 +1,2 @@
 complete -c not -s h -l help -d 'Display help and exit'
-complete -c not -xa '(__fish_complete_subcommand --allow-functions-and-builtins)'
+complete -c not -xa '(__fish_complete_subcommand)'

share/completions/or.fish

@@ -1,3 +1,3 @@
 
 complete -c or -s h -l help -d 'Display help and exit'
-complete -c or -xa '(__fish_complete_subcommand --allow-functions-and-builtins)'
+complete -c or -xa '(__fish_complete_subcommand)'

share/completions/while.fish

@@ -1,2 +1,2 @@
 complete -c while -s h -l help -d 'Display help and exit'
-complete -c while -xa '(__fish_complete_subcommand --allow-functions-and-builtins)'
+complete -c while -xa '(__fish_complete_subcommand)'

share/functions/__fish_complete_external_command.fish

@@ -1,3 +1,4 @@
+# TODO: This function is deprecated, figure out a way to tell users.
 function __fish_complete_external_command
     complete -C "$argv[1]"
 end

share/functions/__fish_complete_subcommand.fish

@@ -1,12 +1,10 @@
 function __fish_complete_subcommand -d "Complete subcommand" --no-scope-shadowing
     # Pass --commandline to complete the remainder of the arguments instead of the commandline.
-    # Pass --allow-functions-and-builtins to enable the completion of the first token as function or builtin.
     # Other args are considered flags to the supercommand that require an option.
 
     # How many non-option tokens we skip in the input commandline before completing the subcommand
     # Usually 1; for ssh 2.
     set -l skip_next 1
-    set -l allow_functions_and_builtins false
     set -l subcommand
     while string match -rq -- '^--[a-z]' $argv[1]
         set -l arg $argv[1]
@@ -14,8 +12,6 @@ function __fish_complete_subcommand -d "Complete subcommand" --no-scope-shadowin
         switch $arg
             case '--fcs-skip=*'
                 set skip_next (string split = -- $arg)[2]
-            case --allow-functions-and-builtins
-                set allow_functions_and_builtins true
             case --commandline
                 set subcommand $argv
                 set -e argv
@@ -48,10 +44,5 @@ function __fish_complete_subcommand -d "Complete subcommand" --no-scope-shadowin
         end
     end
 
-    if test $allow_functions_and_builtins = false && test (count $subcommand) -eq 1
-        __fish_complete_external_command "$subcommand"
-    else
-        printf "%s\n" (complete -C "$subcommand")
-    end
-
+    printf "%s\n" (complete -C "$subcommand")
 end