Remove GSettings use from web process

Our scheme to sync settings from the host to the web process extension has failed, because it depended on the web process's ability to read host settings: i.e. its design never actually worked, since the whole purpose was to avoid depending on host settings, which are inaccessible inside the sandbox.

Instead of fixing this, we can just not allow the web process to read Epiphany settings at all. Toolkit level settings should go through either XSettings or xdg-desktop-portal. Add an assertion to ensure the web process crashes if it tries to use ephy_settings_get() to avoid mistakes here.

Currently we use two settings: do-not-track, and remember-passwords. We can remove the do-not-track setting and just always strip tracking query parameters from URLs, especially since this setting is hidden since 3.34 anyway. In the future, we'll want to remove this code and let ITP do its thing instead. As for remember-passwords, we can manually sync the state of this setting to the web process using D-Bus.

Finally, remove the unused browser_mode variable.

Fixes #993 (closed)