Followup fixes for transaction_timeout

Followup fixes for transaction_timeout

Don't deal with transaction timeout in PostgresMain().  Instead, release
transaction timeout activated by StartTransaction() in
CommitTransaction()/AbortTransaction()/PrepareTransaction().  Deal with both
enabling and disabling transaction timeout in assign_transaction_timeout().

Also, remove potentially flaky timeouts-long isolation test, which has no
guarantees to pass on slow/busy machines.

Reported-by: Andres Freund
Discussion: https://postgr.es/m/20240215230856.pc6k57tqxt7fhldm%40awork3.anarazel.de