babeld: add del_filters function

[PolynomialDivision]

This commit adds the ubus function to dynamically delete filters on runtime:

ubus call babeld del_filters '{"ifname":"eth0"}'

Maintainer: me, @kerneis Compile tested: belkin rt3200 Run tested: belkin rt3200

[BKPepe]

May I know which is your use case or if you are using this anyhow these days? Code was reviewed that's ok, but babeld's developer said that adding these two function to upstream does not make any sense as it is not used, but you are using this here.

So, I think this can go through, though, and it will give time to babeld's developer to come up with something better, as he said.