xl: remove unimplemented -l stub for cpupool-list

xl: remove unimplemented -l stub for cpupool-list

Although advertised via the usage output, xl cpupool-list -l just
returns ERROR_NI, which does not show up on the console. Instead the
output is empty, which is not exactly what --long hints to.  To avoid
confusion remove the line from the help output and just ignore the -l
option properly until it gets finally implemented.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>