kernel: do not fetch git tags

kernel: do not fetch git tags

After a recent jenkins/plugins update, jenkins started listing tags and
attempting git rev-parse ^{commit} on them:

  Seen branch in repository origin/wip-tighter-types
  Seen 39 remote branches
  > git tag -l # timeout=10
  > git rev-parse refs/tags/v4.7-rc7^{commit} # timeout=10
  > git rev-parse refs/tags/v2.6.30-rc7^{commit} # timeout=10

On the Linux kernel repo, this eventually fails with:

  > git rev-parse refs/tags/v2.6.11^{commit} # timeout=10
  FATAL: Command "git rev-parse refs/tags/v2.6.11^{commit}" returned status code 128:
  stdout: refs/tags/v2.6.11^{commit}
  stderr: error: refs/tags/v2.6.11^{commit}: expected commit type, but
  the object dereferences to tree type

Only "trigger" jobs seem to be affected, but we don't really need tags,
so don't fetch them for both kernel-trigger and kernel.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>