Remove Bag-Size from example bag-info.txt

This field is not specified but its presence in the examples can cause
confusion that it's a standard field. Since any app which needs a
human-formatted total file size may trivially calculate it from the
Payload-Oxum (also see #2) or filesystem metadata this commit removes it
to avoid confusion or the implication that tools must maintain the
value.

bagit.xml

@@ -564,7 +564,6 @@ conform to this format:
          FOO papers colle...
     Bagging-Date: 2008-01-15
     External-Identifier: university_foo_001
-    Bag-Size: 260 GB
     Payload-Oxum: 279164409832.1198
     Bag-Group-Identifier: univerisity_foo
     Bag-Count: 1 of 15