kbuild: remove support for single %.symtypes build rule

This rule is unnecessary because you can generate foo/bar.symtypes
as a side effect using:

  $ make KBUILD_SYMTYPES=1 foo/bar.o

While compiling *.o is slower than preprocessing, the impact is
negligible. I prioritize keeping the code simpler.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>