spmi: pmic-arb: revert "disassociate old virq if hwirq mapping already exists"

Now that spmi-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.

Driver was tested using gpio-keys and iadc/vadc on the LG Nexus 5
(hammerhead) phone.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>