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/gui_gtk_x11.c b/src/gui_gtk_x11.c
index a5af3fa..b2770ca 100644
--- a/src/gui_gtk_x11.c
+++ b/src/gui_gtk_x11.c
@@ -1211,15 +1211,16 @@
     if (len == 0)   // Unrecognized key
 	return TRUE;
 
-    // Handle modifiers.
-    modifiers = modifiers_gdk2vim(state);
-
     // For some keys a shift modifier is translated into another key code.
     if (len == -3)
 	key = TO_SPECIAL(string[1], string[2]);
     else
 	key = string[0];
 
+    // Handle modifiers.
+    modifiers = modifiers_gdk2vim(state);
+
+    // Recognize special keys.
     key = simplify_key(key, &modifiers);
     if (key == CSI)
 	key = K_CSI;
@@ -1235,6 +1236,10 @@
 	// <C-H> and <C-h> mean the same thing, always use "H"
 	if ((modifiers & MOD_MASK_CTRL) && ASCII_ISALPHA(key))
 	    key = TOUPPER_ASC(key);
+
+	// May remove the shift modifier if it's included in the key.
+	modifiers = may_remove_shift_modifier(modifiers, key);
+
 	string[0] = key;
 	len = 1;
     }