gluon-core: remove obsolete proto 'batadv' -> 'gluon_mesh' migration

More migration code from 2016.

package/gluon-core/luasrc/lib/gluon/upgrade/800-migrate-batadv

-#!/usr/bin/lua
-
-local uci = require('simple-uci').cursor()
-
-local function migrate_iface(iface)
-	if iface.proto ~= 'batadv' or iface.mesh ~= 'bat0' then
-		return
-	end
-
-	local s = iface['.name']
-
-	uci:set('network', s, 'proto', 'gluon_mesh')
-	uci:set('network', s, 'fixed_mtu', true)
-
-	if iface.mesh_no_rebroadcast then
-		uci:set('network', s, 'transitive', iface.mesh_no_rebroadcast)
-	end
-
-	uci:delete('network', s, 'mesh')
-	uci:delete('network', s, 'mesh_no_rebroadcast')
-end
-
-uci:foreach('network', 'interface', migrate_iface)
-uci:save('network')