refactor(fetch): drop bzr support

[ShellWen]

We plan to remove Python 2 support in AOSC; however, bzr depends on Python 2.

bzr is barely used and unmaintained. Currently, there are no packages in abbs with bzr in SRCS, so we can safely remove bzr support.

[what-the-diff]

PR Summary

• Elimination of Bazaar Functionality
  The functions that managed operations for Bazaar, a version control system, have been entirely removed from the file known as fetch.py. This signals that the support for Bazaar has ceased.

• Modification of Handlers
  All references to 'BZR' in a significant record in fetch.py have been eliminated. This move ensures the cessation of any Bazaar support.

• Documentation Amendments
  The corresponding documentation, found in the appendix.rst file, was updated to exclude any mentions of Bazaar. This includes details regarding its support status and any associated prefixes.

[MingcongBai]

NAK. A compatible alternative is available as Breezy.