No need to re-check IsHeadlessModeRequested in get_desktop_environment

It is already checked on all call chains:

* CreateSalInstance (vcl/unx/generic/plugadapt/salplug.cxx) checks
  IsHeadlessModeRequested before calling autodetect_plugin ->
  get_desktop_environment -> get_desktop_environment

* Application::GetDesktopEnvironment (vcl/source/app/svapp.cxx) checks
  IsHeadlessModeEnabled before potentially calling SalGetDesktopEnvironment
  (vcl/unx/generic/plugadapt/salplug.cxx) -> get_desktop_environment
  -> get_desktop_environment

* The two calls to SalGetDesktopEnvironment in vcl/unx/gtk/ are clearly only done
  for GTK plugins, not headless

Change-Id: I44b43cecd63effc5b2e3ad2a449943fee183462a