Remove test-content subcommand from mach (servo#39412)

Content tests have been replaced by web platform tests 10 years ago (see
servo@9be71b9).
I think it is fair to assume that no one is going to run the relevant
subcommand anymore. Let's remove it.

Signed-off-by: Simon Wülker <[email protected]>

Author: simonwuelker

python/servo/testing_commands.py

@@ -268,11 +268,6 @@ def test_unit(
         assert isinstance(result, int)
         return result
 
-    @Command("test-content", description="Run the content tests", category="testing")
-    def test_content(self) -> int:
-        print("Content tests have been replaced by web-platform-tests under tests/wpt/mozilla/.")
-        return 0
-
     @Command("test-tidy", description="Run the source code tidiness check", category="testing")
     @CommandArgument(
         "--all",