parisc: vdso: remove unused C build rule in vdso32/Makefile

The build rule for C is unused because 'obj-cvdso32' is not defined
in this Makefile.

If you add a C file into arch/parisc/kernel/vdso32/ in the future,
you can revert this commit. The kernel does not keep unused code.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Helge Deller <deller@gmx.de>