docs/library: Remove "only" directive from all pyb module docs.

By virtue of its name, the pyb module would only be available on a pyboard
and so does not need to have conditional "only" directives throughout its
documentation.

These conditionals were added mostly in
cfcf47c0 in the initial development of the
cc3200 port, which had the pyb module before it switched to the machine
module.  And wipy only conditionals were removed from the pyb module
documentation in 45426430, so there's no
need to retain any more conditionals.