This patch implements an improvment that I've wanted to do for ages, but
never gotten around to.

Unused imports are computed based on how imported entities occur (qualified,
unqualified).   This info was accumulated in tcg_used_rdrnames :: Set RdrName.
But that was a huge pain, and it got worse when we introduced duplicate
record fields.

The Right Thing is to record tcg_used_gres :: [GlobalRdrElt], which records
the GRE *after* filtering with pickGREs.  See Note [GRE filtering] in RdrName.
This is much, much bette.  This patch deletes quite a bit of code, and is
conceptually much easier to follow.

Hooray.  There should be no change in functionality.