L2cap: Remove l2cu_can_allocate_lcb()

Checking maximum number of L2cap LCB is not needed because when link is
established, we check and disconnect if maximum number is reached. Also
the condition could change when a classic link is established after the
check is done.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I94b04125f4304937f1c9bc9eb21612685cb76816