sh: fix smp-shx3 build regression from removal of arch localtimer

The removal was not complete and left behind one reference to a
removed function in smp-shx3.c. For completeness, also remove
declarations for functions that were removed.

Fixes: 45624ac3 "sh: remove arch-specific localtimer and use generic one"
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Rich Felker <dalias@libc.org>