wmaker: removed global variable "flags.nopolling"
As found by Rodolfo García Peñas, this flag is never given a value; further
investigations in the history of the project show that this flag have never
been implemented because its action is totally redundant with the flag
"noupdate".
As the later flag's name is more clear about what the behaviour for the
user is, as opposed to what is being done under the hood, its name is kept
and the "nopolling" flag is removed.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>