rust: remove abandoned heapsize crate

Summary: This also unblocks the MacOS Mononoke builds, so enabling them back

Reviewed By: farnz

Differential Revision: D21455422

fbshipit-source-id: 4eae10785db5b93b1167f580a1c887ee4c8a96a2

build/fbcode_builder/manifests/mononoke

@@ -7,10 +7,10 @@ shipit_fbcode_builder = true
 [git]
 repo_url = https://github.com/facebookexperimental/eden.git
 
-[build.os=linux]
+[build.not(os=windows)]
 builder = cargo
 
-[build.not(os=linux)]
+[build.os=windows]
 # building Mononoke on windows is not supported
 builder = nop