Remove vendor from git

Previously we saved the vendor tree in version control, making any
commit that changed a dependency rather large.
Go Modules gives us most of the advantages of vendoring except that if a
dependency which is not stored on the proxy is deleted we would lose
access to it.
For now, we can remove the vendor tree and when we get CI working again
we can possibly generate and save the vendor tree as a build artifact.

Signed-off-by: Sam Whited <sam@samwhited.com>