RCU: Remove prototype for nonexistent function synchronize_idle()

synchronize_idle() sounds like an interesting function, but we don't
actually have it, so don't prototype it.  Introduced in commit
9b06e818, in 2005.

Signed-off-by: Josh Triplett <josh@kernel.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>