Bug 12839: aqbooksellers.gstreg is never used

Bug 12839: aqbooksellers.gstreg is never used

The aqbooksellers.gstreg is never used in the code.
This patch does not remove the DB field but 3 useless occurrences in the
neworderempty page.

The both variable applygst and gstreg have never been took into account for prices calculation.

Test plan:
Verify there is no difference before and after the patch in the prices
calculation.

Signed-off-by: Paola Rossi <paola.rossi@cineca.it>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>