Remove clear variable from disk CheckCommand

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".

Author: RincewindsHat

doc/10-icinga-template-library.md

@@ -414,7 +414,6 @@ disk\_partition		| **Optional.** The partition. **Deprecated in 2.3.**
 disk\_partition\_excluded  | **Optional.** The excluded partition. **Deprecated in 2.3.**
 disk\_partitions 	| **Optional.** The partition(s). Multiple partitions must be defined as array.
 disk\_partitions\_excluded | **Optional.** The excluded partition(s). Multiple partitions must be defined as array.
-disk\_clear             | **Optional.** Clear thresholds. May be true or false.
 disk\_exact\_match      | **Optional.** For paths or partitions specified with -p, only check for exact paths. May be true or false.
 disk\_errors\_only      | **Optional.** Display only devices/mountpoints with errors. May be true or false.
 disk\_ignore\_reserved  | **Optional.** If set, account root-reserved blocks are not accounted for freespace in perfdata. May be true or false.

itl/command-plugins.conf

@@ -1636,6 +1636,9 @@ object CheckCommand "disk" {
 	command = [ PluginDir + "/check_disk" ]
 
 	arguments = {
+		/*
+		"-C" (disk_clear) is missing on purpose, since there is no useful use case possible the way check_disk is mapped here
+		*/
 		"--extra-opts" = {
 			value = "$disk_extra_opts$"
 			description = "Read extra plugin options from an ini file."
@@ -1689,10 +1692,6 @@ object CheckCommand "disk" {
 			key = "-x"
 			value = "$disk_partition_excluded$"
 		}
-		"-C" = {
-			set_if = "$disk_clear$"
-			description = "Clear thresholds"
-		}
 		"-E" = {
 			set_if = "$disk_exact_match$"
 			description = "For paths or partitions specified with -p, only check for exact paths"