ARCHBOM-1305: remove deprecated flag_undefined_default (#24426)

This is the final step in removing the deprecated
flag_undefined_default as explained by the following ADR:
https://github.com/edx/edx-platform/blob/master/openedx/core/djangoapps/waffle_utils/docs/decisions/0001-refactor-waffle-flag-default.rst

Notes:

* All uses of flag_undefined_default=False were always
  supposed to have been no-ops.
* All uses of flag_undefined_default=True that are removed
  in this PR have been replaced by migrations in past PRs.
* The temporary metric temp_flag_default_used id no longer
  reporting any data.

ARCHBOM-1305