txn: drop TXN_CONFLICTED transaction status

The status isn't used anywhere - to set the proper error when an aborted
transaction is attempted to be used, we check out transaction flags
(TXN_IS_CONFLICTED, TXN_IS_ABORTED_BY_YIELD, TXN_IS_ABORTED_BY_TIMEOUT).
Let's use TXN_ABORTED instead.

While we are at it, also set the transaction status to TXN_ABORTED when
a transaction is aborted by yield or timeout and use it instead of
checking flags where appropriate, since it's more convenient.

Follow-up #8123

NO_DOC=code cleanup
NO_TEST=code cleanup
NO_CHANGELOG=code cleanup