wasm: remove heap allocator from wasi-libc

This would conflict with our own heap. We previously defined all those
functions to make sure it's not used, but with a more recent wasi-libc
version (https://github.com/WebAssembly/wasi-libc/pull/250) we can
simply not compile the wasi-libc heap, which is the proper fix.

.circleci/config.yml

@@ -85,10 +85,10 @@ commands:
       - run: go install -tags=llvm<<parameters.llvm>> .
       - restore_cache:
           keys:
-            - wasi-libc-sysroot-systemclang-v3
+            - wasi-libc-sysroot-systemclang-v4
       - run: make wasi-libc
       - save_cache:
-          key: wasi-libc-sysroot-systemclang-v3
+          key: wasi-libc-sysroot-systemclang-v4
           paths:
             - lib/wasi-libc/sysroot
       - run: make gen-device -j4