Better way to avoid unlinking a non-existent file.

This solves a race condition too (although there is still another race),
and is more consistent with similar patterns elsewhere in the file.

Testing Done:
Relevant integration tests pass (after cleaning, so this triggers both a failed unlink and a successful one).

Manually tested the relevant code block.

Reviewed at https://rbcommons.com/s/twitter/r/1438/
Get rid of option_helpers.py.

It was redundant with global_options.py.

This allowed us to make doc gen for global options
more uniform with that for task-specific options.

Testing Done:
CI passed.

Manually generated docs and eyeballed them.

Reviewed at https://rbcommons.com/s/twitter/r/1439/
Sync of https://github.com/pantsbuild/pants/commit/07d517a88c330102d10753b1ad118c22c335737c