Remove getPrimaryPhysicalDisplayId

SurfaceFlinger marks the first connected display as primary, which is
not really useful, and surfaceflinger should not care about this.

The display manager can decide whether a display is primary when needed.

Bug: 240703637
Test: atest libgui_test
Change-Id: Ie33fda8958da7985ceb17526f15a4ab6c6b19403