Deprecate BzrDir.generate_backup_name and use osutils.available_backup_name.

* bzrlib/tests/test_bzrdir.py:
(TestGenerateBackupName.test_deprecated_generate_backup_name): Added.

* bzrlib/workingtree.py:
(WorkingTree.remove.backup): Fix line too long in a call site

* bzrlib/bzrdir.py:
(BzrDir.generate_backup_name): Deperecated.
(BzrDir._available_backup_name): Renamed to better hint about race
conditions and implementation delegated to
osutils.available_backup_name.
(BzrDir.backup_bzrdir): Cleaned up.