Revert "Emergency fix: remove username/password in Elastic Agent (#637)" (#640)

This reverts commit 0e97cbe.

Author: mtojek

internal/install/application_configuration_yml.go

@@ -27,10 +27,6 @@ const applicationConfigurationYml = `stack:
       kibana: ` + kibanaImageName + `@sha256:78ae3b1ca09efee242d2c77597dfab18670e984adb96c2407ec03fe07ceca4f6`
 */
 
-// Emergency fix for: https://github.com/elastic/beats/issues/29603
 const applicationConfigurationYml = `stack:
   image_ref_overrides:
-    8.0.0-SNAPSHOT:
-      # Use stable image versions for Agent/Fleet Server
-      elastic-agent: ` + elasticAgentCompleteImageName + `:8.0.0-52f8db06-SNAPSHOT
 `