Remove shared ptr workarounds

This pull request removes two obsolete or unnecessary workarounds.
1) The workaround to hide assembler code in boost::shared_ptr from cling (header boostWorkAround.h) is no longer needed, because all affected uses of boost::shared_ptr have already been replaced by std::shared_ptr.
2) The ROOT 5 workaround to hide std::shared_ptr from gccxml (header HideStdSharedPtrFromRoot.h)
is irrelevant in ROOT6.  It was automatically forward ported from the main 7_2_X branch, but is not needed here.
Please merge promptly, unless there are issues.