Remove parser_error

It was unused.

src/parser.h

@@ -139,18 +139,6 @@ struct breakpoint_block_t : public block_t {
     breakpoint_block_t();
 };
 
-/// Errors that can be generated by the parser.
-enum parser_error {
-    /// No error.
-    NO_ERR = 0,
-    /// An error in the syntax.
-    SYNTAX_ERROR,
-    /// Error occured while evaluating commands.
-    EVAL_ERROR,
-    /// Error while evaluating cmdsubst.
-    CMDSUBST_ERROR,
-};
-
 struct profile_item_t {
     /// Time spent executing the specified command, including parse time for nested blocks.
     int exec;