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

This commit introduced a test failure in gdb.server/attach-flag.exp.
I didn't spot this failure originally as the problem is fixed by this,
as yet unpushed patch:

  https://sourceware.org/pipermail/gdb-patches/2021-November/183768.html

I unfortunately didn't test each patch in the original series
independently.  I'll repost this patch after the above patch has been
merged.

This reverts commit 32b1f5e8d6b8ddd3be6e471c26dd85a1dac31dda.