Build/CMake: disable unity builds for external dependencies as their support for unity builds was rather hacky and relied too much on random factors

dep/fmt/CMakeLists.txt

@@ -61,12 +61,3 @@ set_target_properties(fmt
   PROPERTIES
     FOLDER
       "dep")
-
-if(UNITY_BUILDS)
-  set_target_properties(fmt
-      PROPERTIES UNITY_BUILD ON)
-  set_target_properties(fmt
-      PROPERTIES
-        UNITY_BUILD_MODE BATCH
-        UNITY_BUILD_BATCH_SIZE 8)
-endif()