Revert the changes in [1] that added the new "http-pipelining" option.

The issue that triggered addition of this option - serf issue 135: ssl
renegotiation breaks when there are pipelined requests - has been solved
in serf directly.
More specifically: serf 1.4 will automatically disable http pipelining for
connections to a server that initiates a SSL renegotiation.

[1] r1619072, r1619074, r1619079.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1621180 13f79535-47bb-0310-9956-ffa450edef68