remove obsolete slice methods

__delslice__() and __setslice__() are deprecated as of python v2.6 [1].
Since urwid does not support python < v2.6 anymore it should be okay to
remove them. The functionality is now provided by __delitem__() and
__setitem__().

Removing them saves some work making MonitoredFocusList support negative
(slice) indices. If not removed, they need to be patched too since
python 2.6 will still call these methods if present [1].

[1] https://docs.python.org/2.6/reference/datamodel.html