Remove unnecessary null pointer checks

[Extra null pointer checks are not needed](https://isocpp.org/wiki/faq/freestore-mgmt#delete-handles-null) in the member function "[ofApp::unloadTextures](https://github.com/ofZach/inkSpace/blob/5ef1fc3bc3f081bdad760fc71b4e6883155aeb5a/src/ofApp.cpp#L975)".
