arch/powerpc/lib/extable.c: sparse fix

extable.c:66:9: warning: symbol 'ex_tab_message' was not declared. Should it be static?

making it static can produce a new build warning on some boards:

extable.c:66:12: warning: 'ex_tab_message' defined but not used [-Wunused-variable]

but ex_tab_message doesn't do much even when used, so just remove it.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>