MINOR: cfgparse-quic: activate pacing only via burst argument

MINOR: cfgparse-quic: activate pacing only via burst argument

Recently, pacing support was added for cubic congestion algorithm. This
was activated by using the new token "cubic-pacing" on quic-cc-algo.
Furthermore, it was possible to define a burst size with a new
parameters after congestion token between parenthesis.

This configuration is not oblivious to users. In particular, it can
cause to easily forgot to tweak burst size, which can dramatically
impact performance.

Simplify this by removing the extra "-pacing" suffix. Now, pacing will
be activated solely based on the burst parameter. If 0, burst is
considered as infinite and no pacing will be used. Pacing will be
activating for any positive burst. This better reflects the link between
pacing and burst and its importance.

Note that for the moment, if burst is specified, it will be ignored with
a warning for algorithm outside of cubic.

This is not a breaking change as pacing support was implemented in the
current dev version.