powerpc: Eliminate unused do_gtod variable

Since we started using the generic timekeeping code, we haven't had a
powerpc-specific version of do_gettimeofday, and hence there is now
nothing that reads the do_gtod variable in arch/powerpc/kernel/time.c.
This therefore removes it and the code that sets it.

Signed-off-by: Paul Mackerras <paulus@samba.org>