Remove dependencies on Boost.StaticAssert.

Boost.StaticAssert has been merged into Boost.Config, so remove
the dependency.

Author: Lastique

appveyor.yml

@@ -71,7 +71,6 @@ install:
   - git submodule update --init libs/headers
   - git submodule update --init libs/config
   - git submodule update --init libs/core
-  - git submodule update --init libs/static_assert
   - git submodule update --init libs/throw_exception
   - xcopy /s /e /q %APPVEYOR_BUILD_FOLDER% libs\assert\
   - cmd /c bootstrap