uml: remove unused sigcontext accessors

The macros which extract registers from a struct sigcontext are no longer
needed and can be removed.  They are starting not to build anyway, given the
removal of the 'e' and 'r' from register names during the x86 merge.

Cc: Jiri Olsa <olsajiri@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>