kbuild: do not create intermediate *.tar for tar packages

Commit 05e96e96 ("kbuild: use git-archive for source package
creation") split the compression as a separate step to factor out
the common build rules.

With the previous commit, we got back to the situation where source
tarballs are compressed on-the-fly.
There is no reason to keep the separate compression rules.

Generate the comressed tar packages directly.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>