This refactors the CWalletTx::RelayWalletTransaction() function to be
clearer and adds comments. It also makes two minor behavior
changes:

• no longer assert if fBroadcastTransactions is false. Just return false

from the function.

• no longer print the relay message if p2pEnabled is set to false (since

the transaction is not actually relayed).

Backport of Core PR15728