mfd: pm8xxx: revert "disassociate old virq if hwirq mapping already exists"

Now that ssbi-gpio is a proper hierarchical IRQ chip, and all in-tree
users of device tree have been updated, we can now drop the hack that
was introduced to disassociate the old Linux virq if a hwirq mapping
already exists. That patch was introduced to not break git bisect for
any existing boards.

This change was tested on an APQ8060 DragonBoard.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>