libsnapshot: Remove disabled /metadata unmount test.

This test should really be run in recovery, but we don't really have any
way to run gtests in recovery. The corresponding C++ code is not really
complicated enough to warrant dependency injection.

Instead, let's remove this test and defer to functional testing of adb
sideload/wipe-during-merge.

Bug: 350715463
Test: N/A
Change-Id: Ic2d206ec51b4324a9570bbf75947236f7577def0