Start cleaning up RemoteKeyboard QML code

Summary:
The code for visually disabling the input field is not needed any more since the field is completely hidden when not needed. Also drop the custom available property from the text field.

More cleanups will follow. My long-term goal is to reuse the code in the app to enable remote keyboard input on non-Plasma DEs

Test Plan: (De)activate the remote keyboard on Android.

Reviewers: KDE Connect, apol

Reviewed By: KDE Connect, apol

Subscribers: apol, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D12669