Drop deprecated \brief Doxygen directive

\brief is a stupid feature that we should stop using:
https://lists.llvm.org/pipermail/llvm-dev/2015-May/085152.html

tests/Unit/MarkForSerializationTest.cpp

@@ -1,5 +1,5 @@
 /// \file MarkForSerializationTest.cpp
-/// \brief Test for MarkForSerialization
+/// Test for MarkForSerialization
 
 //
 // Copyright rev.ng Srls. See LICENSE.md for details.