build: remove dead code

Remove the supporting code for WITH_PTHREAD_WORKQUEUES which was removed
previously.  NFC.

Signed-off-by: Daniel A. Steffen <dsteffen@apple.com>
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index ac2b30a..4ce0e33 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -119,11 +119,6 @@
                              ${CMAKE_CURRENT_SOURCE_DIR}
                              ${CMAKE_CURRENT_BINARY_DIR}
                              ${CMAKE_SOURCE_DIR}/private)
-if(WITH_PTHREAD_WORKQUEUES)
-  target_include_directories(dispatch
-                             SYSTEM BEFORE PRIVATE
-                               "${WITH_PTHREAD_WORKQUEUES}/include")
-endif()
 if(WITH_BLOCKS_RUNTIME)
   target_include_directories(dispatch
                              SYSTEM BEFORE PRIVATE
@@ -174,9 +169,6 @@
   target_link_libraries(dispatch PRIVATE ${BSD_OVERLAY_LDFLAGS})
 endif()
 target_link_libraries(dispatch PRIVATE Threads::Threads)
-if(WITH_PTHREAD_WORKQUEUES)
-  target_link_libraries(dispatch PRIVATE PTHREAD::workqueue)
-endif()
 if(WITH_BLOCKS_RUNTIME)
   target_link_libraries(dispatch PRIVATE BlocksRuntime)
 endif()