Skip old file cleanup for non-full builds.

There are few build targets that don't generate an installed file list,
e.g. product-graph. Skip the old installed file cleanup step so that the
func doesn't complain about a lack of the list file.

Fixes: 168105598
Test: m product-graph
Change-Id: Ib7dce6b801979bb565b74d6355143bac23b84fe6