For the same reason with MUT_ARR_PTRS_CLEAN we don't need to scavenge
SMALL_MUT_ARR_PTRS_CLEAN in mut_lists.

Because SMALL_MUT_ARR_PTRS doesn't have a card table we don't have a
special case when scavenging SMALL_MUT_ARR_PTRS_DIRTY in a mut_list.

Test Plan: this validates

Reviewers: simonmar, bgamari, erikd

Reviewed By: simonmar, bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4800