wireprotov2: remove support for sending bytes response
We recently declared that all responses must be CBOR. So remove
support for sending a type that isn't CBOR data.
Differential Revision: https://phab.mercurial-scm.org/D3387
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL