DOS: skip desktop mode restoration to fix crash when exiting on pure VGA

[palxex]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

This PR fixes a crash on DOS platforms when destroying a window on pure VGA hardware (e.g., IBM ISA VGA in 86box) that occurs 100%.

Testing Env: 86box with IBM ISA VGA, CWSDPMI r7, DJGPP v2.05, GCC 12.2.0.

Crash stack trace( SDL part only, symbolized):

i586-pc-msdosdjgpp-addr2line	function
SDL/src/video/dos/SDL_dosmodes.c:497	DOSVESA_SetDisplayMode
SDL/src/video/SDL_video.c:1553	SDL_SetDisplayModeForDisplay
SDL/src/video/SDL_video.c:2098	SDL_UpdateFullscreenMode
SDL/src/video/SDL_video.c:4544	SDL_DestroyWindow

During SDL_DestroyWindow, SDL_UpdateFullscreenMode is called with SDL_FULLSCREEN_OP_LEAVE. Inside it, there is a SDL_SetDisplayModeForDisplay(display, NULL) call explicitly commented as "Restore the desktop mode". On DOS, this operation is unnecessary during window destruction, and can cause a crash on pure VGA hardware. The actual return to text mode is already handled later in DOSVESA_VideoQuit.
This fix resolves the crash on pure VGA hardware and has been verified to cause no regression on VESA-compatible hardware.