Remove disable_object_pool flag in WM server.

ObjectPool and ObjectPoolItem have been removed,
we no longer need this bug fix flag.
No CTS tests reference this flag, confirming safe removal.

Bug: 311089192
Change-Id: Ie3ece4bc137e6a264b90c450a3d4865636526e13
Test: Build pass, boot success
Flag: EXEMPT removing com.android.window.flags.disable_object_pool