patch 8.2.1231: MS-Windows: GUI code can be cleaned up

Problem:    MS-Windows: GUI code can be cleaned up.
Solution:   Do a bit of cleaning up. (Ken Takata, closes #6465)
diff --git a/src/gui_w32.c b/src/gui_w32.c
index 30ce3ea..46358dc 100644
--- a/src/gui_w32.c
+++ b/src/gui_w32.c
@@ -240,6 +240,7 @@
 # define CONST
 # define FAR
 # define NEAR
+# define WINAPI
 # undef _cdecl
 # define _cdecl
 typedef int BOOL;
@@ -320,9 +321,6 @@
 						// for find/replace dialog
 #endif
 
-#if !defined(FEAT_GUI)
-static
-#endif
 HWND			s_hwnd = NULL;
 static HDC		s_hdc = NULL;
 static HBRUSH		s_brush = NULL;
@@ -389,7 +387,7 @@
 #endif
 
 // use of WindowProc depends on Global IME
-#define MyWindowProc vim_WindowProc
+static LRESULT WINAPI MyWindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam);
 
 extern int current_font_height;	    // this is in os_mswin.c
 
@@ -1259,12 +1257,8 @@
     }
 }
 
-#ifdef PROTO
-typedef int WINAPI;
-#endif
-
-    LRESULT WINAPI
-vim_WindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
+    static LRESULT WINAPI
+MyWindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
 {
 #ifdef GLOBAL_IME
     return global_ime_DefWindowProc(hwnd, message, wParam, lParam);
@@ -3824,10 +3818,6 @@
 # include "xpm_w32.h"
 #endif
 
-#ifdef PROTO
-# define WINAPI
-#endif
-
 #ifdef __MINGW32__
 /*
  * Add a lot of missing defines.