Remove an unused parameter from discover_mimic.
Don't discover mimics using Ash/antennae detection.

This takes away the whole point of a mimic, which is that it presents
a surprise when you find that it is not actually a real item/feature.
Instead, you go down some stairs, or have a new square come into view,
notice a square of smoke, glance down at the message log, and say "Huh,
there was a mimic of something somewhere."