cmd/tailscale: remove dep on clientupdate package if feature is omitted

We already had a featuretag for clientupdate, but the CLI wasn't using
it, making the "minbox" build (minimal combined tailscaled + CLI
build) larger than necessary.

Updates #12614

Change-Id: Idd7546c67dece7078f25b8f2ae9886f58d599002
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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