Prepare takes context

Also remove PrepareEx. It's primary usage was for context. Supplying
parameter OIDs is unnecessary when you can type cast in the query SQL.
If it does become necessary or desirable to add options back it can be
added in a backwards compatible way by adding a varargs as last
argument.

batch_test.go

@@ -165,7 +165,7 @@ func TestConnBeginBatchWithPreparedStatement(t *testing.T) {
 	conn := mustConnectString(t, os.Getenv("PGX_TEST_DATABASE"))
 	defer closeConn(t, conn)
 
-	_, err := conn.Prepare("ps1", "select n from generate_series(0,$1::int) n")
+	_, err := conn.Prepare(context.Background(), "ps1", "select n from generate_series(0,$1::int) n")
 	if err != nil {
 		t.Fatal(err)
 	}