[cmake] Remove find_package(Threads).

Applications/DataExplorer/DataExplorer.cmake

@@ -34,7 +34,6 @@ target_link_libraries(
     QtDataView
     QtDiagramView
     VtkVis
-    $<$<TARGET_EXISTS:Threads::Threads>:Threads::Threads>
     Qt5::Core
     Qt5::Gui
     Qt5::Widgets

Tests/CMakeLists.txt

@@ -112,7 +112,6 @@ target_link_libraries(
             VTK::FiltersSources
             Boost::boost # TODO move OGSMFrontThermodynamicForcesView to
                          # MathLib?
-            $<$<TARGET_EXISTS:Threads::Threads>:Threads::Threads>
             $<$<TARGET_EXISTS:LIE>:LIE>
             $<$<TARGET_EXISTS:TH2M>:TH2M>
             $<$<TARGET_EXISTS:MPI::MPI_CXX>:MPI::MPI_CXX>

scripts/cmake/Find.cmake

@@ -43,11 +43,6 @@ endif()
 # ##############################################################################
 # Find libraries
 # ##############################################################################
-if(OGS_USE_MFRONT)
-    set(CMAKE_REQUIRE_FIND_PACKAGE_Threads TRUE)
-    set(THREADS_PREFER_PTHREAD_FLAG ON)
-endif()
-find_package(Threads)
 
 # Qt5 library ##
 if(OGS_BUILD_GUI)