Remove outdated bz2 examples from tutorials

[menshikh-iv]

MmReader support only filename as input (not file-like object), but in the old documentation (wiki.rst/dist_lsi.rst) we used file-like object too as input.

Current PR remove this outdated usage from examples.

Based on mailing list post

[piskvorky]

Removing this line doesn't sound right -- we still support bz2!

Just remove the (superfluous) bz2.BZFile wrapper. Dtto below.

[menshikh-iv]

@piskvorky fixed in #1868

[piskvorky]

Why don't we support file-like objects in MmCorpus? That sounds like a bug, I see no reason.

[menshikh-iv]

@piskvorky I see code for support file-like, but also see some buggy behavior, look at example

from gensim.corpora import MmCorpus
import bz2

f = bz2.BZ2File("testcorpus.mm.bz2")
print(f.closed)  # 0
corpus = MmCorpus(f)
print(f.closed)  # 1 ???

for this reason, if we try to read from this, we'll receive an exception suggested in mailing list.

[menshikh-iv]

@piskvorky UPD, I found what's a reason for this behavior:
https://github.com/RaRe-Technologies/gensim/blob/5342153eb4f4b02bb45bfa3951eef8250ac9f6b6/gensim/matutils.py#L1274

in this line, we using with here, that automatically call .close when we exit from with scope, for this reason, after Mmreader.__init__ we closed passed file-descriptor.

This is a bug anyway (because internally we use file_or_filename, instead of simple smart_open -> It was assumed that everything should work for file-like)