ada: Deconstruct build support for ancient MinGW

ada: Deconstruct build support for ancient MinGW

Remove conditional C code for building GNAT with MinGW earlier than 2.0,
which was released in 2007.

gcc/ada/

	* adaint.c: Remove conditional #include directives for old MinGW.
	* cal.c: Always include winsock.h, since it is part of modern
	MinGW.
	* cstreams.c: Remove workaround for old MinGW.
	* expect.c: Remove conditional #include directive for old MinGW.
	* mingw32.h: Remove STD_MINGW and OLD_MINGW declarations.
	* sysdep.c: Remove conditional #include directive for old MinGW.