libsnapshot: Removed the unused "linear" optimization.

VAB has an unused optimization that allows bypassing snapshots for the
area of a partition that grows during an OTA. The code for this is
entirely unused since the optimization was never enabled. The benefits
are marginal, and making it safe is quite complicated. The "new" region
cannot overlap with any region being relinquished by a shrink operation,
without snapshotting the region that would be overwritten. This would be
burdensome to implement and would minimize space savings.

Let's remove the code related to this optimization until we are
confident we can implement it safely in VABC.

Bug: 177935716
Test: vts_libsnapshot_test
Change-Id: I7d6a68dce57c8a4389ea6bff9f31971276a20db4