Require access to ConnString via Config

A Config object may be created via ParseConfig and then further
modified. Requiring access to the original ConnString via the Config
helps indicate that the Config is the source of truth as to how the
connection was actually established.

conn.go

@@ -424,9 +424,6 @@ func (c *Conn) StatementCache() stmtcache.Cache { return c.stmtcache }
 // ConnInfo returns the connection info used for this connection.
 func (c *Conn) ConnInfo() *pgtype.ConnInfo { return c.connInfo }
 
-// ConnString returns the connection string that was used to establish this connection.
-func (c *Conn) ConnString() string { return c.config.ConnString() }
-
 // Config returns config that was used to establish this connection.
 func (c *Conn) Config() *ConnConfig { return c.config }