Remove patch for SDL1 sources

We don't patch external projects before builds.

INSTALL

@@ -2,10 +2,6 @@ DOSBox uses the following libraries:
 
 SDL
     The Simple DirectMedia Library available at https://www.libsdl.org/
-    The dll distributed with the windows version of DOSBox is slightly
-    modified. You can find the changes in the sourcepackage of DOSBox
-    (src/platform/sdl-win32.diff). If you want the patched sourcetree
-    send us an email. (see README)
     Note that only version 1.2 and its subversions (1.2.8, 1.2.13 etc.)
     are currently supported.
     License: LGPLv2+

src/platform/Makefile.am

@@ -1,3 +1 @@
 SUBDIRS = visualc
-
-EXTRA_DIST = sdl-win32.diff

src/platform/sdl-win32.diff

@@ -1,55 +0,0 @@
-diff -ru SDL-1.2.12/src/main/win32/version.rc SDL-1.2.12release/src/main/win32/version.rc
---- SDL-1.2.12/src/main/win32/version.rc	2007-07-20 07:52:05.000000000 +0200
-+++ SDL-1.2.12release/src/main/win32/version.rc	2007-08-17 19:03:42.000000000 +0200
-@@ -13,7 +13,7 @@
-  FILEVERSION 1,2,12,0
-  PRODUCTVERSION 1,2,12,0
-  FILEFLAGSMASK 0x3fL
-- FILEFLAGS 0x0L
-+ FILEFLAGS 0x4L
-  FILEOS 0x40004L
-  FILETYPE 0x2L
-  FILESUBTYPE 0x0L
-diff -ru SDL-1.2.12/src/video/SDL_video.c SDL-1.2.12release/src/video/SDL_video.c
---- SDL-1.2.12/src/video/SDL_video.c	2007-07-20 07:52:25.000000000 +0200
-+++ SDL-1.2.12release/src/video/SDL_video.c	2007-08-17 02:51:52.000000000 +0200
-@@ -75,12 +75,12 @@
- #if SDL_VIDEO_DRIVER_GAPI
- 	&GAPI_bootstrap,
- #endif
--#if SDL_VIDEO_DRIVER_WINDIB
--	&WINDIB_bootstrap,
--#endif
- #if SDL_VIDEO_DRIVER_DDRAW
- 	&DIRECTX_bootstrap,
- #endif
-+#if SDL_VIDEO_DRIVER_WINDIB
-+	&WINDIB_bootstrap,
-+#endif
- #if SDL_VIDEO_DRIVER_BWINDOW
- 	&BWINDOW_bootstrap,
- #endif
-diff -ru SDL-1.2.12/src/video/windx5/SDL_dx5events.c SDL-1.2.12release/src/video/windx5/SDL_dx5events.c
---- SDL-1.2.12/src/video/windx5/SDL_dx5events.c	2007-07-20 07:52:25.000000000 +0200
-+++ SDL-1.2.12release/src/video/windx5/SDL_dx5events.c	2007-08-17 02:51:52.000000000 +0200
-@@ -519,7 +519,7 @@
- 		case WM_SYSKEYDOWN: {
- 			/* Pass syskey to DefWindwoProc (ALT-F4, etc.) */
- 		}
--		break;
-+//		break;
- 		case WM_KEYUP:
- 		case WM_KEYDOWN: {
- 			/* Ignore windows keyboard messages */;
-diff -ru SDL-1.2.12/src/video/windx5/SDL_dx5video.c SDL-1.2.12release/src/video/windx5/SDL_dx5video.c
---- SDL-1.2.12/src/video/windx5/SDL_dx5video.c	2007-07-20 07:52:25.000000000 +0200
-+++ SDL-1.2.12release/src/video/windx5/SDL_dx5video.c	2007-08-17 02:51:52.000000000 +0200
-@@ -1496,7 +1496,7 @@
- 			}
- 		}
- 		dd_surface3 = NULL;
--#if 0 /* FIXME: enable this when SDL consistently reports lost surfaces */
-+#if 1 /* FIXME: enable this when SDL consistently reports lost surfaces */
- 		if ( (flags & SDL_HWSURFACE) == SDL_HWSURFACE ) {
- 			video->flags |= SDL_HWSURFACE;
- 		} else {