Remove unused MadviseState hints

The only MadviseState enum value that is used is MadviseStateAtLoad.
Remove the enum entirely in favor of explicit load semantics for the
OatDexFile::MadviseDexFile call.

Follow-up work will address some of the existing madvise behavior for
low-RAM devices, particularly as it relates to the more recently added
madvise knobs like dalvik.vm.madvise.vdexfile.size.

Bug: 196052575
Test: test-art-host
Change-Id: I88211efce9193dc4c45274957898ce7b94eb330c