tx: Remove tx_broadcast transaction from the pool (#4050)
This PR ensures that broadcast future cleans-up the submitted extrinsic
from the pool, iff the `broadcast_stop` operation has been called.
This effectively cleans-up transactions from the pool when the
`broadcast_stop` is called.
cc @paritytech/subxt-team
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Signed-off-by: Alexandru Vasile <[email protected]>