[backport#15680] [rpc] remove resendwallettransactions RPC

Summary:
This RPC was added for testing wallet rebroadcasts. Since we now have a
real test for wallet rebroadcasts, it's no longer needed.

The call in wallet_basic.py can be removed because
wallet_resendwallettransactions.py tests wallet rebroadcast.

https://github.com/bitcoin/bitcoin/pull/15680/commits/f5162458cd3e757ee6bbc7e963b61d365ccc536a

This is a partial backport of Core PR15680

Test Plan:

ninja check
./test_runner.py wallet_basic

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5951