RemoveUnusedBrs: Remove final block nops in all cases

[kripken]

This fixes what looks like it might be a regression in #4943. It's not actually
an issue since it just affects wat files, but it did uncover an existing
inefficiency. The situation is this:

(block
  ..
  (br $somewhere)
  (nop)
)

Removing such a nop is always helpful, as the pass might see that that
br goes to where control flow is going anyhow, and the nop would
confuse it. We used to remove such nops only when the block had a name,
which is why wat testcases looks optimal, but we were actually doing the
less-efficient thing on real-world code. It was a minor inefficiency, though, as
the nop is quickly removed by later passes anyhow. Still, the fix is trivial (to
always remove such nops, regardless of a name on the block or not).