uml: remove unused variables in the context switcher

This patch removes a variable which was not used in two functions.  Yet
another code cleanup, nothing really significant.

Please note that I could not test this on x86_64. I don't have the
hardware for it.

[ jdike - Bits of tidying around the affected code.  Also, it's fine on
x86_64 ]

Signed-off-by: Karol Swietlicki <magotari@gmail.com>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>