i386: remove unused rdtsc() macro

All users to the two-part rdtsc() macro have already switched to using
rdtscl() or rdtscll().  Remove the now-obsolete macro.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>