[backport#15728] [wallet] Refactor CWalletTx::RelayWalletTransaction()

Summary:
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

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6406