Remove PMS dropNonSystemPackages

This seems to have been added in error. A filterOnlySystemPackages
wrapper already exists, which performs the same function.

There are packages which are not required to be preinstalled system
apps, and using the non-specific dropNonSystemPackages wrapper would
accidentally strip those packages from the known set.

Bug: 192915574

Test: presubmit

Change-Id: I490c331653a2aac210ec090d1b5a96a7126f9fa4