remove duplicate SHOW_NOTIFICATIONS_FILTER option (#4481)

The option was removed in https://github.com/tuskyapp/Tusky/pull/3877,
then forgotten to be added again in
https://github.com/tuskyapp/Tusky/pull/4015.
https://github.com/tuskyapp/Tusky/pull/4026 added it again, but took to
long to merge, so we made https://github.com/tuskyapp/Tusky/pull/4225 as
well and that is how we ended up with the option twice.

app/src/main/java/com/keylesspalace/tusky/components/preference/PreferencesFragment.kt

@@ -220,13 +220,6 @@ class PreferencesFragment : PreferenceFragmentCompat() {
                     isSingleLineTitle = false
                 }
 
-                switchPreference {
-                    setDefaultValue(true)
-                    key = PrefKeys.SHOW_NOTIFICATIONS_FILTER
-                    setTitle(R.string.pref_title_show_notifications_filter)
-                    isSingleLineTitle = false
-                }
-
                 switchPreference {
                     setDefaultValue(true)
                     key = PrefKeys.CONFIRM_REBLOGS