should not restore builtin_inline_index

`builtin_inline_index` is restored because THEN clause on
`Primitive.mandatory_only?` was compiled twice.

However, f29c9d6d36 skips to compile THEN clause so we don't
need to restore `builtin_inline_index`.

compile.c

@@ -8348,15 +8348,12 @@ compile_builtin_mandatory_only_method(rb_iseq_t *iseq, const NODE *node, const N
         .script_lines = ISEQ_BODY(iseq)->variable.script_lines,
     };
 
-    int prev_inline_index = GET_VM()->builtin_inline_index;
-
     ISEQ_BODY(iseq)->mandatory_only_iseq =
       rb_iseq_new_with_opt(&ast, rb_iseq_base_label(iseq),
                            rb_iseq_path(iseq), rb_iseq_realpath(iseq),
                            nd_line(line_node), NULL, 0,
                            ISEQ_TYPE_METHOD, ISEQ_COMPILE_DATA(iseq)->option);
 
-    GET_VM()->builtin_inline_index = prev_inline_index;
     ALLOCV_END(idtmp);
     return COMPILE_OK;
 }