wgengine/magicsock: replace CanPMTUD() with ShouldPMTUD()

Replace CanPMTUD() with ShouldPMTUD() to check if peer path MTU discovery should
be enabled, in preparation for adding support for enabling/disabling peer MTU
dynamically.

Updated #311

Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/peermtu_unix.go

@@ -40,7 +40,3 @@ func (c *Conn) connControl(network string, fn func(fd uintptr)) error {
 	}
 	return rc.Control(fn)
 }
-
-func CanPMTUD() bool {
-	return debugEnablePMTUD()
-}