It is not located in Share at all. The contentModel.xml is part of the Repository (backend) and as such located inside the alfresco-repository-x.y.jar in the webapps/alfresco/WEB-INF/lib/ folder. The source of it can be found in GitHub, though due to various migrations / relocations, it will be hard to find the exact state for 5.1. The contentModel.xml should generally not be modified - any reasonable requirement should be solveable by using clean customisation patterns via custom models / behaviours etc.
You can also find the outofthebox content mdoel in "alfresco-repository-5.x.x.jar" file in the exploded war at this location: <ALF_INSTALL_DIR>\tomcat\webapps\alfresco\WEB-INF\lib\alfresco-repository-5.x.x.jar\alfresco\model\
Please note that, you should never make any modifications/alternations to the out of the boxcontent-model.xml file.