[wptrunner] Don't override HeadlessShellBrowser.make_command() (#47399)
#47229 changed the implementation of
ChromeBrowser.port so that it's dynamically set from a free port that
chromedriver finds. This broke HeadlessShellBrowser, which tries to
use the port number before it's available 0.
As it turns out, there's no need for HeadlessShellBrowser to override
ChromeBrowser.make_command().