CMake: Remove unused LINK_IN_PHYSX

dtool/dtool_config.h.in

@@ -233,9 +233,6 @@
    to compile them out.  */
 #cmakedefine NOTIFY_DEBUG
 
-/* Define if we are linking PANDAPHYSX in with PANDA. */
-#cmakedefine LINK_IN_PHYSX
-
 /* The compiled-in character(s) to expect to separate different
    components of a path list (e.g. $PRC_PATH). */
 #define DEFAULT_PATHSEP "@DEFAULT_PATHSEP@"

panda/metalibs/panda/CMakeLists.txt

@@ -11,14 +11,6 @@ if(HAVE_FREETYPE)
   list(APPEND PANDA_LINK_TARGETS p3pnmtext)
 endif()
 
-if(LINK_IN_PHYSX)
-  add_definitions(-DBUILDING_PANDAPHYSX)
-  set(PANDA_LINK_TARGETS
-    ${PANDA_LINK_TARGETS}
-    p3physx
-  )
-endif()
-
 add_metalib(panda panda.cxx COMPONENTS ${PANDA_LINK_TARGETS})
 set_target_properties(panda PROPERTIES DEFINE_SYMBOL BUILDING_LIBPANDA)