chaincfg: remove bad dnsseed (seed.bitcoinstats.com)

seed.bitcoinstats.com doesn't respect filter requests even though it's
advertised as it does. Plus it often returns nodes that are unreachable.
In my testing over the past week, it's been the most unreliable dns seed
and since we have plenty of mainnet seeds, it's better to do away with
this.

I documented my findings for this in a Bitcoin Core issue:
bitcoin/bitcoin#29911

chaincfg/params.go

@@ -285,7 +285,6 @@ var MainNetParams = Params{
 		{"seed.bitcoin.sipa.be", true},
 		{"dnsseed.bluematt.me", true},
 		{"dnsseed.bitcoin.dashjr.org", false},
-		{"seed.bitcoinstats.com", true},
 		{"seed.bitnodes.io", false},
 		{"seed.bitcoin.jonasschnelli.ch", true},
 		{"seed.btc.petertodd.net", true},