targets prune --by-tag adjustment

[mike-scott]

Let's say there are 2 targets with the following tags:

• xyz-1: postmerge
• xyz-2: postmerge,promoted

When the following command is used:
fioctl targets prune --by-tag postmerge

Expected behavior would be to drop the "postmerge" tag on xyz-2 instead of ignoring the target.

So we would end up with 1 target with the following tag:

• xyz-2: promoted

Currently no action is taken on xyz-2 so we're left with:

• xyz-2: postmerge,promoted