Revert "tools/file_import: restore sys.modules. Closes #976"

This reverts commit b48e782dd6.

artiq/tools.py

@@ -6,7 +6,6 @@ import collections
 import atexit
 import string
 import os
-from copy import copy
 
 import numpy as np
 
@@ -84,13 +83,9 @@ def file_import(filename, prefix="file_import_"):
 
     path = os.path.dirname(os.path.realpath(filename))
     sys.path.insert(0, path)
-    try:
-        previous_modules = copy(sys.modules)
     try:
         loader = importlib.machinery.SourceFileLoader(modname, filename)
         module = loader.load_module()
-        finally:
-            sys.modules = previous_modules
     finally:
         sys.path.remove(path)