You're right about the page fault bit, my mistake, a reference count would be sufficient there, the reverse map would be for changing the backing for purposes of swap or migration in a NUMA or HIGHMEM system where quick knowledge page<-->active sets is required.
Reverse mapping for file backed pages don't need anon_vma stuff obviously, because they're not anon. What they use isn't simpler though, as, for example, a library will often be shared by more address spaces than an anon page, it makes more sense to use a search tree (at least this used to be the case, according to a slightly dated early 2.6.x book detailing it. If it's now simpler than this, that would show kernel devs in fact simplifying things, not adding complexity).
If you use none of these, go into your kernel conf and disable CONFIG_SWAP, CONFIG_NUMA and any CONFIG_HIGHMEM entries. If nothing else uses it, it'll either be removed, or worst case, a few greps will show you where the functions are being used elsewhere to throw a few #ifdef's around (I imagine embedded comunity would have interest in already doing this).