[NEXUS-5607] Mark Min to keep / Retention days fields as mandatory in Snapshot removal task

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #825 from sonatype/nexus-5607-snap-remove-mandatory

TRIVIAL: [NEXUS-5607] Mark Min to keep / Retention days fields as mandatory in Snapshot removal task
No code change, reformat.

And removal of unused imports.
Merge remote-tracking branch 'origin/master' into nexus-5511-improve-infex-reasoning

Conflicts:
	nexus-core/src/main/java/org/sonatype/nexus/proxy/storage/local/fs/DefaultFSPeer.java