Remove resolver.c:maybe_destroy()

Remove resolver.c:maybe_destroy()

After refactoring of `validated()`, the `maybe_destroy()` function is
no longer expected to actually destroy the fetch context when it is
being called, so effectively it only ensures that the validators are
canceled when the context has no more queries and pending events, but
that is redundant, because `maybe_destroy()` `REQUIRE`s that the context
should be in the shutting down state, and the function which sets that
state is already canceling the validators in its own turn.

As a failsafe, to make sure that no validators will be created after
`fctx_doshutdown()` is called, add an early return from `valcreate()` if
the context is in the shutting down state.