Remove the system-logs input (#42328) (#42386)

The system-logs input was created to support reading syslog and auth
logs from Linux hosts by the system module, but at the end it was
decided not to use it.

This commit removes its code and references.
---------

Co-authored-by: Denis <[email protected]>
(cherry picked from commit fd9570d)

Co-authored-by: Tiago Queiroz <[email protected]>

CHANGELOG-developer.next.asciidoc

@@ -71,6 +71,7 @@ The list below covers the major changes between 7.0.0-rc2 and main only.
 - Rename x-pack/filebeat websocket input to streaming. {issue}40264[40264] {pull}40421[40421]
 - Journald input now calls `journalctl` instead of using `github.com/coreos/go-systemd/[email protected]/sdjournal`, the CGO dependency has been removed from Filebeat {pull}40061[40061]
 - System module events now contain `input.type: systemlogs` instead of `input.type: log` when harvesting log files, however the ingest pipeline sets it back to the original input (log or journald). {pull}41246[41246]
+- The system-logs input is removed because it's not used anymore {pull}42328[42328]
 
 ==== Bugfixes
 

filebeat/input/default-inputs/inputs_linux.go

@@ -19,7 +19,6 @@ package inputs
 
 import (
 	"github.com/elastic/beats/v7/filebeat/input/journald"
-	"github.com/elastic/beats/v7/filebeat/input/systemlogs"
 	v2 "github.com/elastic/beats/v7/filebeat/input/v2"
 	cursor "github.com/elastic/beats/v7/filebeat/input/v2/input-cursor"
 	"github.com/elastic/beats/v7/libbeat/beat"
@@ -38,7 +37,6 @@ func osInputs(info beat.Info, log *logp.Logger, components osComponents) []v2.Pl
 	zeroPlugin := v2.Plugin{}
 	if journald := journald.Plugin(log, components); journald != zeroPlugin {
 		plugins = append(plugins, journald)
-		plugins = append(plugins, systemlogs.PluginV2(log, components))
 	}
 
 	return plugins