Remove duplicate `${PACKAGE}FILES+= cleanup.ksh` line

[ngie]

Summary

This mutes the duplicate target warning emitted via bsd.files.mk each build.

MFC after: 1 week

Test Plan

Before:

$ (cd ../projects/capsicum-test/tests/sys/cddl/zfs/tests/delegate/; make -V'${PACKAGE}FILES'; )                                                                      
make: "/home/ngie/svn/freebsd/base/projects/capsicum-test/share/mk/bsd.files.mk" line 92: warning: duplicate script for target "_testsFILESINS_cleanup.ksh" ignored
make: "/home/ngie/svn/freebsd/base/projects/capsicum-test/share/mk/bsd.files.mk" line 92: warning: using previous script for "_testsFILESINS_cleanup.ksh" defined here
testsFILES

After:

$ (cd tests/sys/cddl/zfs/tests/delegate/; make -V'${PACKAGE}FILES'; )
testsFILES