mmaprototype: move unused helper functions to constraint_unused.go

Previously, constraint.go included a few helper functions intended for future
production use but not currently in use. This commit moves those helpers into
their own dedicated file to make constraint.go smaller.

Author: wenyihu6

pkg/kv/kvserver/allocator/mmaprototype/BUILD.bazel

@@ -9,6 +9,7 @@ go_library(
         "cluster_state_rebalance_stores.go",
         "constraint.go",
         "constraint_matcher.go",
+        "constraint_unused.go",
         "doc.go",
         "load.go",
         "memo_helper.go",