Clean up fgRemoveDeadStore.

Clean up fgRemoveDeadStore.

Remove duplication and a couple of goto's from `fgRemoveDeadStore`.

This part was very confusing:

`
                /* If this is GT_CATCH_ARG saved to a local var don't bother */

                JITDUMP("removing stmt with no side effects\n");

                if (asgNode->gtFlags & GTF_ORDER_SIDEEFF)
                {
                    if (rhsNode->gtOper == GT_CATCH_ARG)
                    {
                        goto EXTRACT_SIDE_EFFECTS;
                    }
                }
`

The `goto` was to the preceding `if` and was useless since the call
to `gtExtractSideEffList(rhsNode, &sideEffList);` couldn't possibly find
side effects because we already checked that
`rhsNode->gtFlags & GTF_SIDE_EFFECT == 0`.

Commit migrated from https://github.com/dotnet/coreclr/commit/dc389a8a3378972b42200ce9644fb482ec477543