FindPythonLibs: Remove PYTHON_INCLUDE_PATH as input

This cmake variable has been deprecated for over a decade, and using it
as an input could potentially cause unexpected results.  We still need
to keep it as an output variable for compatibility though.

Modules/FindPythonLibs.cmake

@@ -150,13 +150,6 @@ foreach(_CURRENT_VERSION ${_Python_VERSIONS})
     PATH_SUFFIXES python${_CURRENT_VERSION}/config
   )
 
-  # For backward compatibility, honour value of PYTHON_INCLUDE_PATH, if
-  # PYTHON_INCLUDE_DIR is not set.
-  if(DEFINED PYTHON_INCLUDE_PATH AND NOT DEFINED PYTHON_INCLUDE_DIR)
-    set(PYTHON_INCLUDE_DIR "${PYTHON_INCLUDE_PATH}" CACHE PATH
-      "Path to where Python.h is found" FORCE)
-  endif()
-
   set(PYTHON_FRAMEWORK_INCLUDES)
   if(Python_FRAMEWORKS AND NOT PYTHON_INCLUDE_DIR)
     foreach(dir ${Python_FRAMEWORKS})