mirror-gerrit: remove redundant declarations

This was found when running staticcheck over the Zoekt codebase.

'page' was init outside the for loop but the value is immediately
overwritten by the value returned by gerrit client.
Value of 'page' is not used outside of the for loop so let's move the
declarations inside.

Change-Id: Ie449814864a5bcdaca7cc4297117c68d0e524f2e
diff --git a/cmd/zoekt-mirror-gerrit/main.go b/cmd/zoekt-mirror-gerrit/main.go
index 1e349ec..b186de7 100644
--- a/cmd/zoekt-mirror-gerrit/main.go
+++ b/cmd/zoekt-mirror-gerrit/main.go
@@ -126,10 +126,9 @@
 	}
 
 	projects := make(map[string]gerrit.ProjectInfo)
-	page := new(map[string]gerrit.ProjectInfo)
 	skip := "0"
 	for {
-		page, _, err = client.Projects.ListProjects(&gerrit.ProjectOptions{Skip: skip})
+		page, _, err := client.Projects.ListProjects(&gerrit.ProjectOptions{Skip: skip})
 		if err != nil {
 			log.Fatalf("ListProjects: %v", err)
 		}