scons: Get rid of the unused "dirname" property of SourceFile.

Change-Id: I7c52f866542057b9f11ba96434f9c6f93ff0ea46
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48121
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -277,10 +277,6 @@ class SourceFile(object, metaclass=SourceMeta):
     def filename(self):
         return str(self.tnode)
 
-    @property
-    def dirname(self):
-        return dirname(self.filename)
-
     @property
     def basename(self):
         return basename(self.filename)