Remove com.android.window.flags.nav_bar_transparent_by_default

The feature was abandoned and replaced by enforce_edge_to_edge. This CL
removes the flag.

Bug: 232195501
Bug: 433525073
Flag: EXEMPT clean up
Test: presubmit
Change-Id: Ia3f534fc4bc8a649a53dc2c0b42754a1d23ddc4e