definitions.rst: remove unused definitions

Not used and also not correctly capitalized for the project name.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>

definitions.rst

 .. |main_project_name| replace:: OpenHarmony
-.. |main_project_name_bold| replace:: **openharmony**
-.. |main_project_tag| replace:: openharmony
 .. |contact_info| replace:: contact@openharmony.io