Cleaning up multicrop flag

The multicrop flag can now be safely deleted since the flag has been
fully rolled out.

Bug: 438989445
Flag: EXEMPT removing com.android.window.flags.multi_crop
Test: Tested by making sure picker is building and working fine after
removing the flag

Change-Id: If0f88fbff6d6e81021b0f05aace5ebdf6c9e29ed