Cleanup bsdiff public API.

The original function used to generate a patch from two files is no
longer used elsewhere so this patch removes it in favour of the newer
version that takes the streams from memory buffers and uses a patch
writer to generate the patch.

Bug: None.
Test: Ran `bsdiff` manually, verified bspatch applied correctly.

Change-Id: I17aac1d3dd405d733ff6dc35abe8e683adea0d4f