all: remove old +build tags

The //go:build syntax was introduced in Go 1.17:

https://go.dev/doc/go1.17#build-lines

gofmt has kept the +build and go:build lines in sync since
then, but enough time has passed. Time to remove them.

Done with:

    perl -i -npe 's,^// \+build.*\n,,' $(git grep -l -F '+build')

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_server_default.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !linux
-// +build !linux
 
 package derp
 

derp/derphttp/websocket.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build linux || js
-// +build linux js
 
 package derphttp