standardize C-like MIT copyright headers throughout fbsource

Summary:
`/*` is the standard throughout open source code. For example, Firefox uses single /*: https://hg.mozilla.org/mozilla-central/file/21d22b2f541258d3d1cf96c7ba5ad73e96e616b5/gfx/ipc/CompositorWidgetVsyncObserver.cpp#l3

In addition, Rust considers `/**` to be a doc comment (similar to Javadoc) and having such a comment at the beginning of the file causes `rustc` to barf.

Note that some JavaScript tooling requires `/**`. This is OK since JavaScript files were not covered by the linter in the first place, but it would be good to have that tooling fixed too.

Reviewed By: zertosh

Differential Revision: D15640366

fbshipit-source-id: b4ed4599071516364d6109720750d6a43304c089

util/SingleWriterValueList.cpp

@@ -1,4 +1,4 @@
-/**
+/*
  * Copyright (c) Facebook, Inc. and its affiliates.
  *
  * This source code is licensed under the MIT license found in the LICENSE

util/SingleWriterValueList.h

@@ -1,4 +1,4 @@
-/**
+/*
  * Copyright (c) Facebook, Inc. and its affiliates.
  *
  * This source code is licensed under the MIT license found in the LICENSE

util/SingleWriterValueListTest.cpp

@@ -1,4 +1,4 @@
-/**
+/*
  * Copyright (c) Facebook, Inc. and its affiliates.
  *
  * This source code is licensed under the MIT license found in the LICENSE