Remove legacy branch images on feature branch

[reliveyy]

The lagacy branch appears when following case happens:

1. Say you have a feature branch foo and it's based on latest master. It changed utils. (So utils:latest__foo will be built)
2. New PRs have been merged into master. Say xud changed.
3. The foo branch is NOT based on latest master. So we diff it with latest master and it will show utils and xud changed. (So utils:latest__foo and xud:latest__foo will be built)
4. Now we rebase foo on latest master. (Only utils:latest__foo will be built now)

Till step 4 we got a legcy image xud:latest__foo left on DockerHub. If we do not delete this image, then the legacy xud will be used first when we run foo branch. It could lead to potential errors.

P.S.

1. The xud script will use branch image first. If it is missing then master image will be used instead.
2. Currently we need to manually remove legacy branch images on DockerHub to avoid such case happening.

I think we should remove legacy branch images on every build on bot side.