MINOR: quic: allow BBR testing without pacing

MINOR: quic: allow BBR testing without pacing

Pacing is activated per bind line via an optional boolean argument of
quic-cc-algo keyword. Contrary to the default usage, pacing is
automatically activated when BBR is chosen. This is because this
algorithm is expected to run on top of pacing, else its behavior is
undefined.

Previously, pacing argument was thus ignored when BBR was selected.
Change this to support explicit deactivation of pacing with it. This
could be useful to test BBR without pacing when debugging some issues.

This should be backported up to 3.1, after a period of observation.

(cherry picked from commit d04e93bc2eb695f35b66233f0f5b80e2e5c2193a)
Signed-off-by: Aurelien DARRAGON <adarragon@haproxy.com>