mmaintegration: remove pkg/kv/kvserver/allocator/mmaprototypehelpers

Previously, all helper functions from
pkg/kv/kvserver/allocator/mmaprototypehelpers were migrated to
pkg/kv/kvserver/mmaintegration. This commit completes the transition by removing
all usages of mmaprototypehelpers to pkg/kv/kvserver/mmaintegration.

Author: wenyihu6

pkg/BUILD.bazel

@@ -1462,7 +1462,6 @@ GO_TARGETS = [
     "//pkg/kv/kvserver/allocator/load:load_test",
     "//pkg/kv/kvserver/allocator/mmaprototype:mmaprototype",
     "//pkg/kv/kvserver/allocator/mmaprototype:mmaprototype_test",
-    "//pkg/kv/kvserver/allocator/mmaprototypehelpers:mmaprototypehelpers",
     "//pkg/kv/kvserver/allocator/plan:plan",
     "//pkg/kv/kvserver/allocator/plan:plan_test",
     "//pkg/kv/kvserver/allocator/storepool:storepool",