[backport#17381 4/5] Clean up nested scope in GetReservedDestination

Summary:
Suggested https://github.com/bitcoin/bitcoin/pull/17304#discussion_r341194391
by Gregory Sanders <gsanders87@gmail.com>

Reason for keeping the return true return false verbosity is that more code
will be added after the ReserveKeyFromKeyPool() call before returning.

https://github.com/bitcoin/bitcoin/pull/17381/commits/bfd826a675445801adec86a469040f3ceb8172ee

Depends on D7418

Partial backport of Core PR17381

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7419