l2c_csm: delete nullptr check

The code after this unconditionally dereferences `p_ccb->p_lcb`, and
`alarm_cancel` does nothing to somehow set this field. Hence, either
this ~entire switch block needs to be conditional on `p_ccb->p_lcb`, or
this NULL check isn't helpful.

Caught by the static analyzer:
> packages/modules/Bluetooth/system/stack/l2cap/l2c_csm.cc:889:27:
warning: Access to field 'pending_ecoc_conn_cnt' results in a
dereference of a null pointer (loaded from field 'p_lcb')
[clang-analyzer-core.NullDereference]

Tags: #stability
Bug: 206470603
Test: TreeHugger
Change-Id: I85394d3e9d0235f8b5ccc2bce5ca5283bc90f7ed