pysendfile[1] was added as an optional dependency but the library
hasn't been maintained and has got no release since 2014.
What is worse, the sendfile implementation is not actually working
since The SendFileIterator class was removed[2]. (Follow-up[3] removed
the remaining reference to the class). The broken implementation has
not been detected because the client is not currently used to upload
contents.
Remove the incomplete implementation to get rid of the dependency on
the unmaintained library.
[1] https://pypi.org/project/pysendfile/
[2] 76c3620c7effb15be577450c276db1e394c2de3e
[3] 0c151d7d7ea00627fad182898958db55b1fef3fd
Closes-Bug: #2062573
Change-Id: Ia4784f59d16660e8d40c0e409f092ac4e46870b4
205ca3336a