cliccl: remove the debug backup commands

[stevendanna]

trafficstars

These commands have been experimental and undocumented since their original development. Many of the commands provide the same functionality as "SHOW BACKUP". The export command provides unique functionality, but it is hard to use correctly.

Further, leaving these in the code has forced us to maintain APIs in the backupccl code that we would have otherwised removed or refactored.

Release justification: Low risk removal of unsupported functionality.

Release note (cli change): The previously-experimental and undocumented debug backup has been removed.

[cockroach-teamcity]

This change is 

[stevendanna]

bors r=dt

[craig[bot]]

Merge conflict.

[stevendanna]

bors r=dt

[craig[bot]]

Build failed:

• Bazel Essential CI (Cockroach)

[stevendanna]

bors retry

[craig[bot]]

Build succeeded:

• Bazel Essential CI (Cockroach)