Win32: Remove support for original MinGW

The original MinGW distribution appears to no longer be maintained and
should not be used.  Anyone still using MinGW should consider switching
to the MinGW-w64 fork or another actively maintained toolchain.
MinGW-w64 supports 64-bit binaries and provides much newer compilers and
Win32 headers.

Fixes #2540

src/win32_platform.h

@@ -83,24 +83,12 @@
 #ifndef WM_COPYGLOBALDATA
  #define WM_COPYGLOBALDATA 0x0049
 #endif
-#ifndef WM_UNICHAR
- #define WM_UNICHAR 0x0109
-#endif
-#ifndef UNICODE_NOCHAR
- #define UNICODE_NOCHAR 0xFFFF
-#endif
 #ifndef WM_DPICHANGED
  #define WM_DPICHANGED 0x02E0
 #endif
-#ifndef GET_XBUTTON_WPARAM
- #define GET_XBUTTON_WPARAM(w) (HIWORD(w))
-#endif
 #ifndef EDS_ROTATEDMODE
  #define EDS_ROTATEDMODE 0x00000004
 #endif
-#ifndef DISPLAY_DEVICE_ACTIVE
- #define DISPLAY_DEVICE_ACTIVE 0x00000001
-#endif
 #ifndef _WIN32_WINNT_WINBLUE
  #define _WIN32_WINNT_WINBLUE 0x0603
 #endif
@@ -113,9 +101,6 @@
 #ifndef USER_DEFAULT_SCREEN_DPI
  #define USER_DEFAULT_SCREEN_DPI 96
 #endif
-#ifndef OCR_HAND
- #define OCR_HAND 32649
-#endif
 
 #if WINVER < 0x0601
 typedef struct