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>

net/tsdial/peerapi_macios_ext.go

@@ -7,8 +7,6 @@
 // and System Extension). It's not used on tailscaled-on-macOS.
 
 //go:build ts_macext && (darwin || ios)
-// +build ts_macext
-// +build darwin ios
 
 package tsdial