Remove `lt-cred-mech` Coturn option

Coturn reports it as an option that conflicts with `use-auth-secret`.
Some reasoning is here: fa523e8d09

roles/matrix-server/templates/coturn/turnserver.conf.j2

@@ -1,4 +1,3 @@
-lt-cred-mech
 use-auth-secret
 static-auth-secret={{ matrix_coturn_turn_static_auth_secret }}
 realm=turn.{{ hostname_matrix }}