gdb: add assert in remote_target::wait relating to async being off

While working on another patch I ended up in a situation where I had
async mode disabled (with 'maint set target-async off'), but the async
event token got marked anyway.

In this situation GDB was continually calling into
remote_target::wait, however, the async token would never become
unmarked as the unmarking is guarded by target_is_async_p.

We could just unconditionally unmark the token, but that would feel
like just ignoring a bug, so, instead, lets assert that if
!target_is_async_p, then the async token should not be marked.

This assertion would have caught my earlier mistake.

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