database/sql: Remove unnecessary call to maybeOpenNewConnections

numCanOpen will never be larger than 0 in maybeOpenNewConnections() when this
code path is taken, so no new connections can ever be opened.

Change-Id: Id1302e8d9afb3a67be61b5e738fe07ef81d20fe0
Reviewed-on: https://go-review.googlesource.com/1550
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>