Performance: disable event counting by default

Disable event counting in the ring buffer, which can count the number of
events produced per ring-buffer, as well as the number of events
overwritten in overwrite mode.

This feature is currently unused anyway: it is not saved in the ring
buffer header, nor made available to lttng-tools.

This saves 70 ns/event in lttng-ust on the ARM32 Cubietruck.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>