GarageBand 10 and GarageBand 5 are installing the loops and the Instrument Library in the same place. It is hard to separate them after you installed GarageBand 10.
This document describes, where loops get installed: https://support.apple.com/en-us/HT201503
Most of the loops are identical, so you will not save much space by deleting the GarageBand 5 loops. And if you delete them, your older projects may no longer work. The new GarageBand 10 loops are needing more storage than the older loops, because GarageBand 10 loops are all real instrument loops that need large sample files. The GarageBand 5 loops, that are not in GB 10 are software instrument loops and do not need much storage.