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

This reverts commit 5f3e417bb5.

artiq/tools.py

@@ -7,7 +7,6 @@ import os
 import atexit
 import string
 import random
-from copy import copy
 
 import numpy as np
 
@@ -85,13 +84,9 @@ 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)