In the general case, I think Gilles answer to the linked Meta discussion is correct.
Generally speaking, if a tag has a well-defined meaning, let it be. Do not remove it (except from questions that it doesn't apply to). Tags that no one cares about are not harmful. Removing a tag that someone cares about is harmful. Do remove tags when they're meaningless (but you should generally discuss the specifics on Meta first) or in favor of another tag with the same meaning (there's a synonym mechanism for that; only remove a tag if it's useless in itself, for example because it's a misspelling).
In the specific question you linked to, I think the resultset tag is appropriate. The OP was asking specifically about the behavior as it pertained to JDBC classes, particularly
ResultSet, so tagging it that way does convey some information.
Who would filter using it? Who would think this is an expertise area?
Not many, but three people do subscribe to the tag. I can imagine using it as a search term if I were learning JDBC and wanted to find everything I could about it.
Is it OK to remove it in order to let more important tags shine?
The more important tags (the ones that are used more often) already shine through. They're listed on the question and in search results in order of how often they're used.