plugins/test: Remove legacy CMake remnants

Some legacy files were missed in 72428ccd97

test/test-input/cmake/legacy.cmake

@@ -1,24 +0,0 @@
-project(test-input)
-
-add_library(test-input MODULE)
-
-target_sources(
-  test-input
-  PRIVATE test-filter.c
-          test-input.c
-          test-sinewave.c
-          sync-async-source.c
-          sync-audio-buffering.c
-          sync-pair-vid.c
-          sync-pair-aud.c
-          test-random.c)
-
-target_link_libraries(test-input PRIVATE OBS::libobs)
-
-if(MSVC)
-  target_link_libraries(test-input PRIVATE OBS::w32-pthreads)
-endif()
-
-set_target_properties(test-input PROPERTIES FOLDER "tests and examples")
-
-setup_plugin_target(test-input)