powerpc/vdso: Remove vdso_setup()

vdso_fixup_features() cannot fail anymore and that's
the only function called by vdso_setup().

vdso_setup() has become trivial and can be removed.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/11522eec6140f510a8c89c63cbb739277d097fdc.1601197618.git.christophe.leroy@csgroup.eu