Remove utils.external from sys.path

Fixes #1952, fixes #1380

- This was added by Pierre a long time ago to use an internal version of
pyflakes but never removed.

spyderlib/utils/external/__init__.py

@@ -11,7 +11,3 @@
 External libraries needed for Spyder to work.
 Put here only untouched libraries, else put them in utils.
 """
-
-import sys
-from spyderlib.baseconfig import get_module_source_path
-sys.path.insert(0, get_module_source_path(__name__))