Require msgpack 3.1.0

Since 3.1.0 msgpack ships a proper CMake package exporting a target
for the header-only C++ library. Allows us to get rid of our custom
find module.

cmake/Findmsgpack.cmake

@@ -1,46 +0,0 @@
-################################################################################
-#    Copyright (C) 2018 GSI Helmholtzzentrum fuer Schwerionenforschung GmbH    #
-#                                                                              #
-#              This software is distributed under the terms of the             #
-#              GNU Lesser General Public Licence (LGPL) version 3,             #
-#                  copied verbatim in the file "LICENSE"                       #
-################################################################################
-
-unset(_args)
-
-if(msgpack_FIND_VERSION)
-  list(APPEND _args ${msgpack_FIND_VERSION})
-endif()
-
-if(msgpack_FIND_EXACT)
-  list(APPEND _args "EXACT")
-endif()
-
-if(msgpack_FIND_QUIETLY)
-  list(APPEND _args "QUIET")
-endif()
-
-if(msgpack_FIND_REQUIRED)
-  list(APPEND _args "REQUIRED")
-endif()
-
-if(msgpack_FIND_COMPONENTS)
-  list(APPEND _args "COMPONENTS" ${msgpack_FIND_COMPONENTS})
-endif()
-
-find_package(msgpack ${_args} CONFIG)
-
-if(msgpack_FOUND AND NOT TARGET msgpack::msgpack)
-  # config mode find_package does not set $msgpack_ROOT, workaround by extracting
-  # root path from library target
-  unset(_msgpack_lib)
-  unset(_prefix)
-  get_target_property(_msgpack_lib msgpackc INTERFACE_LOCATION)
-  get_filename_component(_prefix ${_msgpack_lib} DIRECTORY)
-  get_filename_component(_prefix ${_prefix}/.. ABSOLUTE)
-
-  add_library(msgpack::msgpack INTERFACE IMPORTED)
-  set_target_properties(msgpack::msgpack PROPERTIES
-    INTERFACE_INCLUDE_DIRECTORIES "${_prefix}/include"
-  )
-endif()