database/sql: allow simultaneous queries, etc in a Tx

Now that revision 0c029965805f is in, it's easy
to guarantee that we never access a driver.Conn
concurrently, per the database/sql/driver contract,
so we can remove this overlarge mutex.

Fixes issue 3857

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7707047