UI: Fix `qt-helpers.{c,h}pp` not being removed from legacy.cmake

Follow-up to https://github.com/obsproject/obs-studio/pull/10957

UI/cmake/legacy.cmake

@@ -295,8 +295,6 @@ target_sources(
           multitrack-video-error.hpp
           multitrack-video-output.cpp
           multitrack-video-output.hpp
-          qt-helpers.cpp
-          qt-helpers.hpp
           system-info.hpp)
 
 target_sources(obs PRIVATE importers/importers.cpp importers/importers.hpp importers/classic.cpp importers/sl.cpp