powerpc/vdso: Remove vdso_patches[] and associated functions

vdso_patches[] is now empty, remove it and remove
all functions that depends on it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/27d75debd6e4ddeaffe1d66ffed1e7526684a004.1601197618.git.christophe.leroy@csgroup.eu