Remove WindowContextWindowRmItem pooling (19/n).

Remove the use of ObjectPool in the creation and management of
WindowContextWindowRemovalItem object. Instead of being obtained from
the pool, these objects are now directly instantiated, simplifying their
handling and aligning with the broader removal of the object pooling
mechanism.

Bug: 311089192
Test: atest FrameworksCoreTests:ClientTransactionItemTest
Test: atest FrameworksCoreTests:TransactionExecutorTests
Flag: EXEMPT removing com.android.window.flags.disable_object_pool
Change-Id: Ib12c0bdb66d003dd7faa8dcd2611eefdd2fbd986