cmd/derper: stop setting content header in handleBootstrapDNS

No one really cares. Its cost outweighs its usefulness.

name                   old time/op    new time/op    delta
HandleBootstrapDNS-10     105ns ± 4%      65ns ± 2%   -37.68%  (p=0.000 n=15+14)

name                   old alloc/op   new alloc/op   delta
HandleBootstrapDNS-10      416B ± 0%        0B       -100.00%  (p=0.000 n=15+15)

name                   old allocs/op  new allocs/op  delta
HandleBootstrapDNS-10      3.00 ± 0%      0.00       -100.00%  (p=0.000 n=15+15)

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

cmd/derper/bootstrap_dns.go

@@ -57,7 +57,6 @@ func refreshBootstrapDNS() {
 
 func handleBootstrapDNS(w http.ResponseWriter, r *http.Request) {
 	bootstrapDNSRequests.Add(1)
-	w.Header().Set("Content-Type", "application/json")
 	j, _ := dnsCache.Load().([]byte)
 	w.Write(j)
 }