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