TableGen: use auto and for-range
TableGen: delete no-op code

This does nothing but remove the Record from the map, and
then re-add it, without actually changing it in between.

The Record's Name used to be changed before re-adding it
when the code was first committed in r137232, but the
name-changing lines were removed in r142510, and since
then this code seems to do nothing.

This was also the only caller of removeClass or removeDef,
so now RecordKeeper owns its Records unconditionally,
and could be unique_ptr-ified.
TableGen: unique_ptr-ify RecordKeeper