refactor: remove redundant fOnlySafe argument

Summary:
The fOnlySafe argument to AvailableCoins is now redundant, since #21359
added a similar field inside the CCoinControl struct.

Not all code paths set a CCoinControl instance, but when it's missing we
can default to using only safe inputs which is backwards-compatible.

This is a backport of core#21910

Depends on D14403

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14405