[Plugins] fix experimental status of some plugins

src/plugins/impact/options.ml

@@ -24,7 +24,7 @@ include Plugin.Register
   (struct
      let name = "impact"
      let shortname = "impact"
-     let help = "impact analysis (experimental)"
+     let help = "impact analysis"
    end)
 
 module Pragma =

src/plugins/report/report_parameters.ml

@@ -24,7 +24,7 @@ include Plugin.Register
     (struct
       let name = "report"
       let shortname = "report"
-      let help = "Properties Status Report (experimental)"
+      let help = "Properties Status Report"
     end)
 
 module Print =

src/plugins/server/server_parameters.ml

@@ -28,7 +28,7 @@ module P = Plugin.Register
     (struct
       let name = "Server"
       let shortname = "server"
-      let help = "Frama-C Request Server"
+      let help = "Frama-C Request Server (experimental)"
     end)
 
 include P