Revert "[PATCH] x86_64: Only do the clustered systems have unsynchronized TSC assumption on IBM systems"

This reverts commit 13a229ab.

Quoth Andi:
  "After some consideration and feedback from various people it turns
   out this wasn't that good an idea.  It has some problems and needs
   more work.  Since it was only an optimization anyways it's best to
   just back it out again for now."
Signed-off-by: Linus Torvalds <torvalds@osdl.org>