kbuild: do not remove empty *.symtypes explicitly

Presumably, 'test -s $@ || rm -f $@' intends to remove the output when
the genksyms command fails.

It is unneeded because .DELETE_ON_ERROR automatically removes the output
on failure.

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