- Remove is_default_thread function which is an internal and not used
  function
- Remove g_main_thread_id as it was used only by is_default_thread

Fixes:
 - http://autobuild.buildroot.net/results/5320bbe1205e782e3516d9bead8d1ed825bcbaadSigned-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>