scons: Get rid of special handling of the _m5 package.

This package is handled specially by the DictImporter used during the
build, and an assert shows that that code is never actually used. That
makes sense, since _m5 won't be added to gem5 using the PySource
mechanism.

Change-Id: I36a39f1ebb94a7620c8ba296e0fe856bd33285f9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48362
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>

src/SConscript

@@ -769,9 +769,6 @@ class DictImporter(object):
         if fullname == 'm5.objects':
             return self
 
-        if fullname.startswith('_m5'):
-            return None
-
         source = self.modules.get(fullname, None)
         if source is not None and fullname.startswith('m5.objects'):
             return self