util: simplify listdict

listdict came into the code base from some other python code of mine, for no
apparent reason: it is actually not used here at all. However, an upcoming
patch will use a dict of lists.

Also, the listdict implementation is convoluted/complex (to allow accessing
keys as direct object members, which we don't need). Simplify the
implementation to be used by I0939ef414bc599ee8742df48da04d8d9569d00ba.

Change-Id: I09adfd128a19c6c5ba36aae1d4cab83dbd07e0fb
1 file changed