mruby-compiler: fix as-pattern parsing with symbol values

Remove non-standard `symbol tASSOC p_as` rule from hash pattern
elements. This rule conflicted with the as-pattern rule and caused
`:foo => x` to be incorrectly parsed as a hash pattern instead of
an as-pattern.

CRuby only supports label syntax (foo:) for hash pattern keys,
not hashrocket syntax (:foo =>). This change aligns mruby with
CRuby behavior and reduces bison shift-reduce conflicts from 2 to 1.

Before: `case :foo; in :foo => x; end` raised NoMethodError
After:  `case :foo; in :foo => x; end` binds x to :foo

Co-authored-by: Claude <noreply@anthropic.com>

Author: matz

mrbgems/mruby-compiler/core/parse.y

@@ -4133,6 +4133,7 @@ p_hash_elems    : p_hash_elem
 
 /* Hash pattern element: key: pattern or key: (shorthand) */
 /* Use p_as, not p_expr to avoid brace-less recursion inside hash patterns */
+/* Note: CRuby only supports label syntax (foo:), not hashrocket (:foo =>) */
 p_hash_elem     : tIDENTIFIER tLABEL_TAG p_as
                     {
                       /* {key: pattern} */
@@ -4143,11 +4144,6 @@ p_hash_elem     : tIDENTIFIER tLABEL_TAG p_as
                       /* {key:} shorthand - binds to variable with same name */
                       $$ = cons(new_sym(p, $1), new_pat_var(p, $1));
                     }
-                | symbol tASSOC p_as
-                    {
-                      /* {:"key" => pattern} or {:key => pattern} */
-                      $$ = cons($1, $3);
-                    }
                 ;
 
 /* Keyword rest pattern: **var, **nil, or ** */