wifi: remove hidden unused Hotspot 2.0 Release 2 APIs

These APIs are used for installing/modifying the PerProviderSubscription
Management Object tree in XML string representation and are used only by
the unreleased Hotspot 2.0 Release 2 app (OSU app).  The OSU app will be
updated to use the new APIs which uses Parcelable objects to represent
provider configuration.

Bug: 32509661
Test: frameworks/base/wifi/tests/runtests.sh
Change-Id: Iff6b6e4b95b76d48201eeeb47f441b7c94f359bb