Bug 12512 - PROG/CCSR deprecation: Remove OpacShowFiltersPulldownMobile system preference

CCSR having been deprecated there is no longer a use for the
OpacShowFiltersPulldownMobile system preference. This patch removes
it.

To test, apply the patch and run updatedatabase. Check that the
preference can no longer be found in system preferences.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Preference removed, no koha-qa errors

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>