Clean up trivially redundant __USE_MISC conditionals.

This patch cleans up cases of __USE_MISC that are trivially redundant
after the recent substitution of __USE_MISC for __USE_BSD and
__USE_SVID: either in constructs such as "defined __USE_MISC ||
defined __USE_MISC", or else (in the bits/mman.h case) a conditional
on __USE_MISC nested inside another __USE_MISC conditional.  (The
cleanups remaining after this patch are still quite large, but it
seems a reasonable piece to separate out.)

Tested x86_64.

	* bits/mman.h [__USE_MISC]: Remove redundant conditionals.
	* ctype/ctype.h [__USE_MISC]: Likewise.
	* dirent/dirent.h [__USE_MISC]: Likewise.
	* grp/grp.h [__USE_MISC]: Likewise.
	* io/fcntl.h [__USE_MISC]: Likewise.
	* io/sys/stat.h [__USE_MISC]: Likewise.
	* libio/stdio.h [__USE_MISC]: Likewise.
	* posix/unistd.h [__USE_MISC]: Likewise.
	* pwd/pwd.h [__USE_MISC]: Likewise.
	* stdlib.h [__USE_MISC]: Likewise.
	* string/bits/string2.h [__USE_MISC]: Likewise.
	* string/string.h [__USE_MISC]: Likewise.
	* time/time.h [__USE_MISC]: Likewise.