tools: remove rm -rf vendor/ from make get_vendor_deps

[ValarDragon]

We now ensure that the Gopkg.lock is correct, and that the toml
doesn't depend on any branches, so this is no longer necessary.

Closes #1715

• Updated all relevant documentation (docs/)
• Updated all relevant code comments
• Wrote tests
• Updated CHANGELOG.md
• Updated cmd/gaia and examples/

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #1731 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1731   +/-   ##
========================================
  Coverage    62.35%   62.35%           
========================================
  Files          120      120           
  Lines         7119     7119           
========================================
  Hits          4439     4439           
  Misses        2429     2429           
  Partials       251      251

[cwgoes]

utACK, excellent

Prior to launch we need to go over our dependency security, and possibly audit a few then pin exact versions.