OPTIM: h2: do not update committed streams if elasticity disabled

OPTIM: h2: do not update committed streams if elasticity disabled

When streams-elasticity is enabled in the configuration, H2 mux is
responsible to update the global committed_extra_streams value.

Adjust these operations to ensure they are skipped if streams-elasticity
is disabled, which is the current default. This prevents unnecessary
atomic operations in this case.

No need to backport unless streams-elasticity feature is picked in older
releases.