liblp: Remove last_logical_sector from LpMetadataGeometry.

Now that backup metadata is stored at the start of the super partition,
this field is no longer needed. In actuality, it was not needed even
before then: both it and first_logical_sector exist for convenience,
since they can be re-derived at any time given an LpMetadataGeometry.

Bug: 116802789
Test: liblp_test gtest
      device with dynamic partitions flashes and boots

Change-Id: I259a443097e689a0a9db7f822bbf1a52d40076dc