Remove old/unused Bigtable v1 generated code

[dhermes]

@tseaver there are a lot of import aliases leftover like:

from gcloud.bigtable._generated import (
    bigtable_instance_admin_pb2 as instance_admin_v2_pb2)

Is it worth me tracking them down?

[tseaver]

This PR would be a lot easier to read if broken up: e.g., first remove all the _generate/*.py modules in one PR (so that we could see that there was no breakage / overlap), and then rename _generated_v2 -> _generated in a subsequent PR: at that point, maybe ditching the aliases would be worthwhile.

[dhermes]

@tseaver It's broken up into 3 commits to accomplish essentially the same thing (but a little differently):

1. Remove all v1 code
2. Remove all v2 references in text files
3. Rename all v2 files

[dhermes]

@tseaver PTAL

[tseaver]

@dhermes Your second commit couldn't possibly pass tests: the rename of the files has to go hand-in-hand with the updated imports. Could you squash it with the third commit?

[dhermes]

The failure was due to a newly introduced (hence not renamed) import from #2002.

I made the commits as a group, not trying to get tests to pass in between. I avoided doing the import renames in the same commit as the file renames to make the diff easier to read. Having a "file rename only" commit makes the commit easier to grok.

[tseaver]

OK, LGTM. Looking forward to the day that we can rip this junk out, and just use the grpc-google-{datastore,bigtable}-v# libraries instead.