DRTVWR-476: Introduce LLEventMailDrop::discard() (instead of flush()).

Overriding virtual LLEventPump::flush() for the semantic of discarding
LLEventMailDrop's queued events turns out not to be such a great idea, because
LLEventPumps::flush(), which calls every registered LLEventPump's flush()
method, is called every mainloop tick. The first time we hit a use case in
which we expected LLEventMailDrop to hold queued events across a mainloop tick,
we were baffled that they were never delivered.

Moving that logic to a separate method specific to LLEventMailDrop resolves
that problem. Naming it discard() clarifies its intended functionality.