patch 8.2.1422: the Mac GUI implementation is outdated

Problem:    The Mac GUI implementation is outdated and probably doesn't even
            work.
Solution:   Remove the Mac GUI code.  The MacVim project provides the
            supported Vim GUI version.
diff --git a/src/gui.c b/src/gui.c
index 5d003a3..5ae29fa 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -56,7 +56,7 @@
  * this makes the thumb indicate the part of the text that is shown.  Motif
  * can't do this.
  */
-#if defined(FEAT_GUI_ATHENA) || defined(FEAT_GUI_MAC)
+#if defined(FEAT_GUI_ATHENA)
 # define SCROLL_PAST_END
 #endif
 
@@ -846,7 +846,7 @@
 }
 
 #if defined(FEAT_GUI_GTK) || defined(FEAT_GUI_X11) || defined(FEAT_GUI_MSWIN) \
-	|| defined(FEAT_GUI_PHOTON) || defined(FEAT_GUI_MAC) || defined(PROTO)
+	|| defined(FEAT_GUI_PHOTON) || defined(PROTO)
 # define NEED_GUI_UPDATE_SCREEN 1
 /*
  * Called when the GUI shell is closed by the user.  If there are no changed
@@ -1377,7 +1377,7 @@
 #endif
 
 # if defined(FEAT_GUI_TABLINE) && (defined(FEAT_GUI_MSWIN) \
-	|| defined(FEAT_GUI_MOTIF) || defined(FEAT_GUI_MAC))
+	|| defined(FEAT_GUI_MOTIF))
     if (gui_has_tabline())
 	text_area_y += gui.tabline_height;
 #endif