[ZM_U] Once the Imago cocoon is defeated, the cocoon falls down, destroys the bridge below it and reveals a tunnel. After that, the game clears two blocks at the end of the tunnel.

Unfortunately the game also clears the bg1 value of the two blocks. That looks awkward.

This patch replaces the code that clears the two blocks with a bunch of code that clears only the clipdata of the two blocks. It also saves 40 bytes in imago cocoons ai.