patch 8.2.1019: mapping <M-S-a> does not work in the GUI

Problem:    Mapping <M-S-a> does not work in the GUI.
Solution:   Move the logic to remove the shift modifier to
            may_remove_shift_modifier() and also use it in the GUI.
diff --git a/src/proto/misc2.pro b/src/proto/misc2.pro
index 3477452..e1e20aa 100644
--- a/src/proto/misc2.pro
+++ b/src/proto/misc2.pro
@@ -71,6 +71,7 @@
 int trans_special(char_u **srcp, char_u *dst, int flags, int *did_simplify);
 int special_to_buf(int key, int modifiers, int keycode, char_u *dst);
 int find_special_key(char_u **srcp, int *modp, int flags, int *did_simplify);
+int may_remove_shift_modifier(int modifiers, int key);
 int extract_modifiers(int key, int *modp, int simplify, int *did_simplify);
 int find_special_key_in_table(int c);
 int get_special_key_code(char_u *name);