powerpc/32: Remove CLR_TOP32

CLR_TOP32() is defined as blank. Last useful instance of CLR_TOP32()
was removed by commit 40ef8cbc

 ("powerpc: Get 64-bit configs to
compile with ARCH=powerpc") in 2005.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>