Once x-goog-api-client headers, gccl, is used for storage, remove gcloud-python

[crwilcox]

internal bug: b/143493862

The Python Client has a patch googleapis/google-cloud-python#9548 to temporarily collect metrics until we address a pipeline issue. This is a temporary fix and should be removed once that is done.

[tseaver]

@frankyn Do we still need to preserve the gcloud-python header?

[frankyn]

As far as I'm aware yes. @jsimonweb do you know if we are tracking the correct headers now?

[tseaver]

@frankyn, @crwilcox, @jsimonweb Bumping to check on this one again: should we be removing the gcloud-python header (i.e., revert dc817e9)?