Remove --process-dependency-links from UPGRADE.rst

[aaronraimist]

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file
• Pull request includes a sign off

pip 19.0 finally removed that option https://pip.readthedocs.io/en/latest/news/#id4 so you get an error if you try to use it https://matrix.to/#/!QtykxKocfZaZOUrTwp:matrix.org/$15486036393651Rugay:disroot.org?via=matrix.org&via=half-shot.uk&via=linuxgaming.life

[codecov-io]

Codecov Report

Merging #4485 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #4485   +/-   ##
========================================
  Coverage    74.71%   74.71%           
========================================
  Files          336      336           
  Lines        34102    34102           
  Branches      5544     5544           
========================================
  Hits         25481    25481           
  Misses        7048     7048           
  Partials      1573     1573

[hawkowl]

Thanks!