Can you boot to internet recovery?
That is:
Command-OPTION-R
You will need your wifi password.
It will take some time for the internet utilities to load.
When they do, open disk utility.
You can now erase the entire drive if you want.
My recommendation for getting rid of partitions you don't want:
1. BACK UP the stuff you DO want to save to an external drive.
2. Boot to internet recovery (as above)
3. Use disk utility to ERASE the entire drive (for El Cap, choose Mac OS extended with journaling enabled, GUID partition format)
4. Create any new partitions you want while you are still in disk utility.
5. Now, restore your data from the backup drive.