Remove find_package(PythonInterp)

* find_package(PythonInterp) is deprecated and the set variables
  are anyhow not used anywhere
diff --git a/extra/FindNanopb.cmake b/extra/FindNanopb.cmake
index fb9e1ca..3e1a763 100644
--- a/extra/FindNanopb.cmake
+++ b/extra/FindNanopb.cmake
@@ -335,8 +335,6 @@
 )
 mark_as_advanced(NANOPB_GENERATOR_SOURCE_DIR)
 
-find_package(PythonInterp REQUIRED)
-
 include(FindPackageHandleStandardArgs)
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(NANOPB DEFAULT_MSG
   NANOPB_INCLUDE_DIRS