remove readpst, duplicate of libpst

This commit removes readpst from the main Homebrew repo because
it is a duplicate of libpst.  They both use the same tarball,
same homepage, and compile nearly the same.  The only difference
is that libpst has a dep on boost.  So go with libpst which is
already the latest version.

Closes Homebrew#13089.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>