zstdmt: remove (dupe, unmaintainable)

The user who wrote this code on GitHub has since deleted their account,
making any updates impossible. Furthermore, this package is redundant
anyway: Zstandard has been shipping a compatible 'zstdmt' binary, API,
and stable multi-threading support for over a year now.

Signed-off-by: Austin Seipp <aseipp@pobox.com>