fix(cmake): Remove target compile defs for non-existing QML target

The target `mixxx-qml-lib` does not exist in the 2.4 branch. This is
likely some left-over/copy-and-paste error.

If you build with `-DQT6=ON` (and the `GITHUB_ACTIONS` environment
variable set), this leads to the CMake error:

    CMake Error at CMakeLists.txt:2675 (target_compile_definitions):
      Cannot specify compile definitions for target "mixxx-qml-lib" which is not
      built by this project.

CMakeLists.txt

@@ -2671,9 +2671,6 @@ endif()
 option(DEBUG_ASSERTIONS_FATAL "Fail if debug become true assertions" OFF)
 if(DEBUG_ASSERTIONS_FATAL)
   target_compile_definitions(mixxx-lib PUBLIC MIXXX_DEBUG_ASSERTIONS_FATAL MIXXX_DEBUG_ASSERTIONS_ENABLED)
-  if(QT6)
-    target_compile_definitions(mixxx-qml-lib PUBLIC MIXXX_DEBUG_ASSERTIONS_FATAL MIXXX_DEBUG_ASSERTIONS_ENABLED)
-  endif()
   if (NOT CMAKE_BUILD_TYPE STREQUAL "Debug")
     message(STATUS "DEBUG_ASSERT statements have been enabled because DEBUG_ASSERTIONS_FATAL is ON.")
   endif()