Inspector cleanup

[alexander-yakushev]

1. Remove code responsible for rendering Javadoc fragments (already dropped from cider-nrepl).
2. Remove compatibility code that was introduced together with the javadoc fragment code – use the new approach exclusively. I'm not sure what the compatibility code was for; perhaps, for cases when people relied on those functions in their Emacs config somehow. Anyway, a few years has passed already.

• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the user manual (if adding/changing user-visible functionality)

[bbatsov]

The changes look good to me.

[bbatsov]

We might want to do the OS integration tests just on one version of Emacs, as those are taking forever. I came back to merge this PR only to see the CI is still in progress one hour later.