fix: remove mdns_legacy & Discovery.MDNS.Interval

* fix: remove mdns_legacy

We've been running both implementations for a long, long time.
It is time to remove legacy version and lower the number of LAN packets
IPFS node produces.

See ipfs/kubo#9048 (comment)
for the Interval removal rational.

go.mod

@@ -245,7 +245,6 @@ require (
  github.com/whyrusleeping/cbor-gen v0.0.0-20210219115102-f37d292932f2 // indirect
  github.com/whyrusleeping/chunker v0.0.0-20181014151217-fe64bd25879f // indirect
  github.com/whyrusleeping/go-keyspace v0.0.0-20160322163242-5b898ac5add1 // indirect
- github.com/whyrusleeping/mdns v0.0.0-20190826153040-b9b60ed33aa9 // indirect
  github.com/whyrusleeping/timecache v0.0.0-20160911033111-cfcb2f1abfee // indirect
  go.opentelemetry.io/otel/exporters/otlp/internal/retry v1.7.0 // indirect
  go.opentelemetry.io/otel/exporters/otlp/otlptrace v1.7.0 // indirect

go.sum

@@ -1556,7 +1556,6 @@ github.com/whyrusleeping/go-sysinfo v0.0.0-20190219211824-4a357d4b90b1 h1:ctS9An
 github.com/whyrusleeping/go-sysinfo v0.0.0-20190219211824-4a357d4b90b1/go.mod h1:tKH72zYNt/exx6/5IQO6L9LoQ0rEjd5SbbWaDTs9Zso=
 github.com/whyrusleeping/mafmt v1.2.8/go.mod h1:faQJFPbLSxzD9xpA02ttW/tS9vZykNvXwGvqIpk20FA=
 github.com/whyrusleeping/mdns v0.0.0-20180901202407-ef14215e6b30/go.mod h1:j4l84WPFclQPj320J9gp0XwNKBb3U0zt5CBqjPp22G4=
-github.com/whyrusleeping/mdns v0.0.0-20190826153040-b9b60ed33aa9 h1:Y1/FEOpaCpD21WxrmfeIYCFPuVPRCY2XZTWzTNHGw30=
 github.com/whyrusleeping/mdns v0.0.0-20190826153040-b9b60ed33aa9/go.mod h1:j4l84WPFclQPj320J9gp0XwNKBb3U0zt5CBqjPp22G4=
 github.com/whyrusleeping/multiaddr-filter v0.0.0-20160516205228-e903e4adabd7 h1:E9S12nwJwEOXe2d6gT6qxdvqMnNq+VnSsKPgm2ZZNds=
 github.com/whyrusleeping/multiaddr-filter v0.0.0-20160516205228-e903e4adabd7/go.mod h1:X2c0RVCI1eSUFI8eLcY3c0423ykwiUdxLJtkDvruhjI=