Bug 1637778 - Delete `mach robocop` from py2 mach command list r=nalexander

This isn't an actual `mach` command (it was deleted and replaced by `mochitest`).

Differential Revision: https://phabricator.services.mozilla.com/D75210

mach

@@ -51,7 +51,6 @@ py2commands="
     release-history
     remote
     repackage
-    robocop
     rusttests
     static-analysis
     talos-test