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>

ipn/ipnlocal/peerapi_h2c.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !ios && !android && !js
-// +build !ios,!android,!js
 
 package ipnlocal
 

ipn/ipnlocal/peerapi_macios_ext.go

@@ -3,8 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build ts_macext && (darwin || ios)
-// +build ts_macext
-// +build darwin ios
 
 package ipnlocal
 

ipn/ipnlocal/ssh.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build linux || (darwin && !ios)
-// +build linux darwin,!ios
 
 package ipnlocal
 

ipn/ipnlocal/ssh_test.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build linux || (darwin && !ios)
-// +build linux darwin,!ios
 
 package ipnlocal