wallet: Remove path checking code from loadwallet RPC

Summary:
This commit does not change behavior except for error messages which now
include more complete information.

This is a backport of core#19619 [6/8]
https://github.com/bitcoin/bitcoin/pull/19619/commits/a987438e9d9cad0b5530e218a447928485f3fd93

Depends on D10228
Depends on D10213 (for rpc_createmultisig.py)

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10229