[flutter_plugin_tools] Remove support for bypassing, or prompting for, git tagging

[stuartmorgan-g]

We never want a plugin to be published without tagging the release, so there's no reason to support the added complexity of these flags. Similarly, once someone has confirmed publishing, we don't want to give them an opt-out for doing the tag (and doing so can be harmful; at least once I accidentally typo'd that prompt—"yy" IIRC—and had to manually do the tagging steps to fix it).

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[stuartmorgan-g]

This is just a quick cleanup before I do some more refactoring in this command. If there's some reason I'm missing for having these options, please let me know and I can drop this :)

[cyanglaz]

LGTM!