L2CAP: Remove UCD related flags

* L2CAP UCD (Unicast Connectionless Data) is no longer enabled on the
  stack
* L2CAP_UCD_INCLUDED is always FALSE, the TRUE case should be removed
* As result, L2CAP_UCD_MTU, L2CAP_UCD_IDLE_TIMEOUT, and
  L2CAP_UCD_CH_PRIORITY need to be removed as well

Bug: 66244184
Test: build
Change-Id: Ieee8b3a547653efa3a34a0810c415ba4c32f1be7