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 would 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