kbuild: remove the last use of old cmd_src_tar rule in packaging

The rpm-pkg and deb-pkg targets have transitioned to using 'git archive'
for tarball creation.

Although the old cmd_src_tar is still used by snap-pkg, there is no need
to pack and unpack a tarball solely for passing the source to snapcraft.

Instead, you can use 'source-type: local' to tell the source location to
snapcraft.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/package/snapcraft.template

@@ -10,5 +10,5 @@ parts:
   kernel:
     plugin: kernel
     source: SRCTREE
-    source-type: tar
+    source-type: local
     kernel-with-firmware: false