database/sql: remove extra RemoveDep call
database/sql: remove extra RemoveDep call

This should have been removed in 45c12efb4635. Not a correctness
issue, but unnecessary work.

This CL also adds paranoia checks in removeDep so this doesn't
happen again.

Fixes Issue 5502

R=adg
CC=gobot, golang-dev, google
https://codereview.appspot.com/9543043

firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old