patch 8.1.1306: Borland support is outdated and doesn't work

Problem:    Borland support is outdated and doesn't work.
Solution:   Remove Borland support, there are other (free) compilers
            available. (Thomas Dziedzic, Ken Takata, closes #4364)

src/gui_w32.c

@@ -1303,9 +1303,6 @@ gui_mch_def_colors(void)
     int
 gui_mch_open(void)
 {
-#ifndef SW_SHOWDEFAULT
-# define SW_SHOWDEFAULT 10	/* Borland 5.0 doesn't have it */
-#endif
     /* Actually open the window, if not already visible
      * (may be done already in gui_mch_set_shellsize) */
     if (!IsWindowVisible(s_hwnd))
@@ -3799,9 +3796,6 @@ _OnScroll(
  * Add a lot of missing defines.
  * They are not always missing, we need the #ifndef's.
  */
-# ifndef _cdecl
-#  define _cdecl
-# endif
 # ifndef IsMinimized
 #  define     IsMinimized(hwnd)		IsIconic(hwnd)
 # endif
@@ -5521,7 +5515,7 @@ gui_mch_set_sp_color(guicolor_T color)
  * First static functions (no prototypes generated).
  */
 # ifdef _MSC_VER
-#  include <ime.h>   /* Apparently not needed for Cygwin, MingW or Borland. */
+#  include <ime.h>   /* Apparently not needed for Cygwin or MinGW. */
 # endif
 # include <imm.h>
 
@@ -6272,15 +6266,6 @@ gui_mch_draw_string(
     void
 gui_mch_flush(void)
 {
-#   if defined(__BORLANDC__)
-    /*
-     * The GdiFlush declaration (in Borland C 5.01 <wingdi.h>) is not a
-     * prototype declaration.
-     * The compiler complains if __stdcall is not used in both declarations.
-     */
-    BOOL  __stdcall GdiFlush(void);
-#   endif
-
 #if defined(FEAT_DIRECTX)
     if (IS_ENABLE_DIRECTX())
 	DWriteContext_Flush(s_dwc);