Removing FilesSearch feature flag (#17548)

* Removing FilesSearch feature flag

* Fixing tests

* Adding an improvement on plain text extraction

* Adding tests for plain text extraction

* Removed unneeded conversion

* Adding missed license

* Remove the feature flag from the migration

* Fixing some tests

* Updating i18n/en.json file

api4/file.go

@@ -734,11 +734,6 @@ func searchFiles(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if !c.App.Config().FeatureFlags.FilesSearch {
-		c.Err = model.NewAppError("searchFiles", "api.post.search_files.not_implemented.app_error", nil, "", http.StatusNotImplemented)
-		return
-	}
-
 	if !c.App.SessionHasPermissionToTeam(*c.App.Session(), c.Params.TeamId, model.PERMISSION_VIEW_TEAM) {
 		c.SetPermissionError(model.PERMISSION_VIEW_TEAM)
 		return