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>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f