This commit removes the -C parameter from the disk CheckCommand
since there is no possible way to use it in any functional capacity.
-C (or --clear) would reset the thresholds given previously
to allow for setting different thresholds for following filesystmes.
As an example:
check_disk -w 50% -c 5% -p / -C -w 1% -p /home
would only set the warning threshold for /home.
Since there is no way to use it reasonably with the Icinga 2
implementation of check_disk (since thresholds can only be
given once and the order is undefined), the clear flag
has no worth here.
My suggestion is to remove it avoid suggesting that it might
be used, but I left it as a comment in the ITL to prevent
the next person from "adding a missing parameter".