Bug 1834006 part 2: Remove ATK specific password masking code. r=morgan

The core, cross-platform implementation already masks passwords.
Thus, the ATK specific code has been unnecessary for a long time now.
The ATK masking code also only applied to LocalAccessible.
This means we can get rid of this differentiation between local and remote.

Differential Revision: https://phabricator.services.mozilla.com/D178610