Remove the platform VPN use case from the FGS type "systemExemption"

VPN apps using the platform-based VPN APIs don't need to run a FGS
to keep the VPN active.

Bug: 267820824
Test: atest CtsAppFgsTestCases
Change-Id: I35eed3c55253c2d8b100a2b2c7c2c3445708073f
1 file changed