Revert "Usernames must start with a letter (#5581)"

This reverts commit 2e911b77c3.

store/sql_team_store_test.go

@@ -146,12 +146,10 @@ func TestTeamStoreSearchByName(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if r1 := <-store.Team().SearchByName(o1.Name); r1.Err != nil {
+	if r1 := <-store.Team().SearchByName("zzz"); r1.Err != nil {
 		t.Fatal(r1.Err)
 	} else {
 		if r1.Data.([]*model.Team)[0].ToJson() != o1.ToJson() {
-			t.Log(r1.Data.([]*model.Team)[0].ToJson())
-			t.Log(o1.ToJson())
 			t.Fatal("invalid returned team")
 		}
 	}