Remove the MTA-STS (Strict Transport Security) implementation

This commit removes the experimental MTA-STS (Strict Transport Security)
implementation for now, as it's not up to date with the latest draft.

Development will continue on the "sts" branch, but this way it won't
block releases until it is ready.

Commits reverted:
 - cb6500b993
 - 0eeb964534
 - e66288e4b4
 - 216cf47ffa
 - d66b06de51
 - fe00750e39
 - 933ab54cd8

internal/courier/smtp_test.go

@@ -23,7 +23,7 @@ func newSMTP(t *testing.T) (*SMTP, string) {
 		t.Fatal(err)
 	}
 
-	return &SMTP{dinfo, nil}, dir
+	return &SMTP{dinfo}, dir
 }
 
 // Fake server, to test SMTP out.