Log into OSMC via ssh and delete files that you don’t need. Basically everything under /home/osmc is what you have installed yourself and you can review what you don’t need in there.
Details regarding how to access the command line interface can be found here on our Wiki: Accessing the command line - General - OSMC
Here is a cheatsheet for how to navigate in a Linux shell: Cheatsheets and Tutorials for users new to Linux based operating systems