powerpc: Remove obsolete prototype

The include/asm-powerpc/paca.h file has a prototype for a function that
does not exist any more; its name is setup_boot_paca.  This function was
removed in commit 4ba99b97, so its
prototype should have been removed at that time too.

Signed-off-by: Julio M. Merino Vidal <jmerino@ac.upc.edu>
Signed-off-by: Adrian Bunk <bunk@kernel.org>