gatt: Remove reduntant `in_use` flag

Now, when clcb is dynamic, there is no need to keep in_use flag, as it
is always true when clcb exist.

Bug: 249431973
Test: atest BluetoothInstrumentationTests
Test: manual GATT operation testing
Tag: #feature
Change-Id: I1bfb4c546c41ca590942feab718b597514f8a59e