scons: Get rid of SourceFile's done function.

This function doesn't seem to be used by anything. A similar one off
mechanism exists for SimObjects which is we might also be able to get
rid of.

Change-Id: I65038c3495fa69cf94042dc78f09876d5e9a557f
Reviewed-on: https://gem5-review.googlesource.com/5841
Maintainer: Gabe Black <gabeblack@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

src/SConscript

@@ -152,14 +152,6 @@ class SourceFile(object):
     def __eq__(self, other): return self.filename == other.filename
     def __ne__(self, other): return self.filename != other.filename
 
-    @staticmethod
-    def done():
-        def disabled(cls, name, *ignored):
-            raise RuntimeError("Additional SourceFile '%s'" % name,\
-                  "declared, but targets deps are already fixed.")
-        SourceFile.__init__ = disabled
-
-
 class Source(SourceFile):
     ungrouped_tag = 'No link group'
     source_groups = set()