ImportKeys: Remove "Import all keys" from menu

- This feature is now available in a bottom bar when in basic mode

OpenKeychain/src/main/res/menu/import_keys_file_fragment.xml

@@ -1,8 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<menu xmlns:android="http://schemas.android.com/apk/res/android">
-
-    <item
-        android:id="@+id/import_all_keys"
-        android:title="@string/menu_import_all_keys" />
-
-</menu>