Remove vendored dependencies

For a lot of our projects, vendoring is necessary for packaging.  This
library is never packaged directly, so vendoring is not necessary.

Removing vendored dependencies reduces the size of the repository,
speeding up checkouts, especially shallow ones, and makes PRs that
include dependency changes in addition to code changes much easier to
review.

The main (if not only) downside is that it makes it impossible to build
and test in disconnected environments, which is not a requirement we
have.

Signed-off-by: Achilleas Koutsou <[email protected]>