Bug 1638951 - Run `mach cargo` with Python 3 r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D75999

mach

@@ -14,7 +14,6 @@ py2commands="
     android
     awsy-test
     browsertime
-    cargo
     check-spidermonkey
     clang-format
     cppunittest

python/mozbuild/mozbuild/mach_commands.py

@@ -108,8 +108,7 @@ class CargoProvider(MachCommandBase):
     @Command('cargo', category='build',
              description='Invoke cargo in useful ways.')
     def cargo(self):
-        self.parser.print_usage()
-        return 1
+        pass
 
     @SubCommand('cargo', 'check',
                 description='Run `cargo check` on a given crate.  Defaults to gkrust.')