L2cap: Remove duplicated OnAclDisconnected invocation

This is invoked in two places and could cause crash.

In LinkManagerTest, removed the client callback verification part
because it is already handled in LinkTest.

Test: cert/run --host
Test: atest bluetooth_test_gd
Bug: 157578574
Tag: #gd-refactor
Change-Id: I8ba6f92d4770fedcd968e543b07720a84c215af1