Complete removal of urcu-signal flavor

This commit completes removal of the urcu-signal flavor.

Users can migrate to liburcu-memb with a kernel implementing the
membarrier(2) system call to have similar read-side performance without
requiring use of a reserved signal, and with improved grace period
performance.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I75b9171e705b9b2ef4c8eeabe6164e5587816fb4