Fix RemoveAccesses, delete CSESubAccesses (#2157) (#2159)

CSESubAccesses was intended to be a simple workaround for a quadratic
performance bug in RemoveAccesses but ended up having tricky corner
cases and was hard to get right. The solution to the RemoveAccesses
bug--quadratic expansion of dynamic indexes of vecs of aggreate
type--turned out to be quite simple and makes CSESubAccesses much less
useful and not worth fixing.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit a41af6f0

)

Co-authored-by: Jack Koenig <koenig@sifive.com>