TST/CLN: remove MultiIndex compat

[jreback]

we have these 2 pickles

(pandas) bash-3.2$ ls pandas/tests/indexes/data/
mindex_073.pickle       multiindex_v1.pickle

should blow these away and the older MultiIndex formats. These are really really old.

note will conflict slightly with #21514 (IOW path will be different after)

Hey I'd like to take this one on!