Remove obsolete predictive back system animations developer option

Predictive back system animations are enabled by default since
Android 15. Therefore this developer option is no longer relevant.

Bug: 412572850
Test: Manual, i.e. verified that developer option no longer shows up
Flag: EXEMPT bugfix
Change-Id: I2d3b83d51b2cda5bfa7d99c3948fd2578eddfb5a