Remove obscure "low-latency" parts in the introduction of spec

Remove obscure "low-latency" parts in the introduction of spec

According to Wikipedia
http://en.wikipedia.org/wiki/Latency_%28engineering%29#Packet-switched_networks
latency means "the time from the source sending a packet to the destination
receiving it". Therefore, latency is unrelated to whether the operation is
asynchronous or synchronous. And also unrelated to whether it's one-way or
round-trip. Latency exists for asynchronous and one-way transfer, because for
current DBus implementations we need at least one context switch to transfer
each message from the sender process to the receiver process. Emphasizing
D-Bus is low-latency could encourage user to abuse/misuse the system.

Mail disscusion:
http://lists.freedesktop.org/archives/dbus/2013-May/015665.html

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=65141
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>