GestureConverter: clean up fling stopping flag

This flag launched in 24Q4.

Bug: 347910711
Test: atest --host inputflinger_tests:GestureConverterTest
Test: m checkinput
Flag: EXEMPT removing com.android.input.flags.enable_touchpad_fling_stop
Change-Id: I4e46eec5d34b3835452db00184f7a0c2ccaf1225