libfiemap_writer: Remove Flush and Write methods.

We no longer use these methods since they require write access to the
userdata block device. This patch removes them to simplify FiemapWriter,
and avoid opening userdata with write permissions.

Bug: 122556707
Test: fiemap_writer_test gtest
Change-Id: I0091eb5ff92253533c67c89b0059c34953aae211