pools: Allow disabling pool even if it's the last one enabled.

Pool state machine is incomplete, and enabled_pools is not increased if
pool state is not explicitly set to "enabled" in config. However,
there is no reason why the user should not be allowed to disable
all pools.

enabled_pools is a bookkeeping variable, and bookkeeping is cumbersome.
Rather than trying to fix this regression, get rid of another place
where enabled_pools is used.

Closes https://github.com/veox/sgminer/issues/75