SdkLevelUtil: remove isAtLeastSMockable()

The mockable version can only be used where object construction
is controlled e.g. in Wifi service. For existing public APIs,
SdkLevelUtil cannot be injected because the constructor can't
be changed.

Thus, the strategy for testing this is:
1. Gate the unit tests with the same SDK level requirements,
  and test on a real device with that SDK version, or
2. Use static mocking.

In either case, the mockable version is not useful.

Bug: 169443762
Test: atest FrameworksWifiApiTests
Change-Id: Icf2f32b3a40512f87bf32d1d80622c22e4454406