feature/featuretags, all: add ts_omit_acme to disable TLS cert support

I'd started to do this in the earlier ts_omit_server PR but
decided to split it into this separate PR.

Updates #17128

Change-Id: Ief8823a78d1f7bbb79e64a5cab30a7d0a5d6ff4b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/localapi/localapi.go

@@ -67,7 +67,6 @@ type LocalAPIHandler func(*Handler, http.ResponseWriter, *http.Request)
 // then it's a prefix match.
 var handler = map[string]LocalAPIHandler{
 	// The prefix match handlers end with a slash:
-	"cert/":     (*Handler).serveCert,
 	"profiles/": (*Handler).serveProfiles,
 
 	// The other /localapi/v0/NAME handlers are exact matches and contain only NAME