powerpc/vdso: Stripped VDSO is not needed, don't build it

Since commit 24b659a1 ("powerpc: Use unstripped VDSO image for
more accurate profiling data"), only the unstripped VDSO image
has been used.

Partially revert commit 8150caad ("[POWERPC] powerpc vDSO: install
unstripped copies on disk") to avoid building the stripped version.

And the unstripped version in $(MODLIB)/vdso/ is not required
anymore as it is the one embedded in the kernel image.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5986ca25be44fe6e9790486304507f240077d8c4.1601197618.git.christophe.leroy@csgroup.eu