[libc++] Remove the old HTML documentation

This commit finishes moving the <atomic> design documents to the RST
documentation and removes the old documentation. https://libcxx.llvm.org
is already pointing to the new documentation only now, so the removal of
the old documentation is really a NFC.

I went over the old documentation and I don't think we're leaving anything
important behind - I think everything important was mentionned in the RST
documentation anyway.

Author: ldionne

libcxx/docs/DesignDocs/AtomicDesign.rst

@@ -165,17 +165,18 @@ Design Documents
 .. toctree::
    :maxdepth: 1
 
-   DesignDocs/DebugMode
-   DesignDocs/CapturingConfigInfo
    DesignDocs/ABIVersioning
+   DesignDocs/AtomicDesign
+   DesignDocs/CapturingConfigInfo
+   DesignDocs/DebugMode
    DesignDocs/ExperimentalFeatures
-   DesignDocs/VisibilityMacros
-   DesignDocs/ThreadingSupportAPI
-   DesignDocs/FileTimeType
-   DesignDocs/FeatureTestMacros
    DesignDocs/ExtendedCXX03Support
-   DesignDocs/UniquePtrTrivialAbi
+   DesignDocs/FeatureTestMacros
+   DesignDocs/FileTimeType
    DesignDocs/NoexceptPolicy
+   DesignDocs/ThreadingSupportAPI
+   DesignDocs/UniquePtrTrivialAbi
+   DesignDocs/VisibilityMacros
 
 * `<atomic> design <http://libcxx.llvm.org/atomic_design.html>`_
 * `<type_traits> design <http://libcxx.llvm.org/type_traits_design.html>`_