Allow pool to be destroyed with an extra (erroneous) consumer reference on the pool account (#4503)

addresses https://github.com/paritytech/polkadot-sdk/issues/4440 (will
close once we have this in prod runtimes).
related: https://github.com/paritytech/polkadot-sdk/issues/2037.

An extra consumer reference is preventing pools to be destroyed. When a
pool is ready to be destroyed, we
can safely clear the consumer references if any. Notably, I only check
for one extra consumer reference since that is a known bug. Anything
more indicates possibly another issue and we probably don't want to
silently absorb those errors as well.

After this change, pools with extra consumer reference should be able to
destroy normally.