control/controlknobs,tailcfg,wgengine/magicsock: remove DRPO shutoff switch

The DERP Return Path Optimization (DRPO) is over four years old (and
on by default for over two) and we haven't had problems, so time to
remove the emergency shutoff code (controlknob) which we've never
used. The controlknobs are only meant for new features, to mitigate
risk. But we don't want to keep them forever, as they kinda pollute
the code.

Updates #150

Change-Id: If021bc8fd1b51006d8bddd1ffab639bb1abb0ad1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/debugknobs_stubs.go

@@ -22,8 +22,6 @@ func debugEnableSilentDisco() bool     { return false }
 func debugSendCallMeUnknownPeer() bool { return false }
 func debugPMTUD() bool                 { return false }
 func debugUseDERPAddr() string         { return "" }
-func debugUseDerpRouteEnv() string     { return "" }
-func debugUseDerpRoute() opt.Bool      { return "" }
 func debugEnablePMTUD() opt.Bool       { return "" }
 func debugRingBufferMaxSizeBytes() int { return 0 }
 func inTest() bool                     { return false }