Blender Git Loki

Keymap: disallow modal key-maps in add-ons keyconfig

Disable functionality reported in T60766 & only partially worked.

This could be used if the key-map was added after Blender started
as a way to customize modal key-maps, however it didn't work with
the add-on enabled on startup.

Add-on key-maps are intended to extend existing key-maps
so they can call the add-on, not as a way to change modal key-maps
for Blender's built-in functionality.

Disable this since it's not needed as add-ons
can't yet define modal key-maps.