libsnapshot: Remove flaky image creation test.

This test has always been flaky, and is not testing something super
valuable: we know that image creation succeeds throughout the rest of
the suite, so it's not very interesting to know that it can succeed in a
low-space scenario.

The inverse test is much more valuable, since we want the correct status
code when creation fails due to low space.

Bug: 240391002
Test: vts_libsnapshot_test
Change-Id: I6235d11033d2f30efe530077b877863ba2574810