arm: sofcpga: s10: remove unused ad-hoc reset code

The stratix 10 reset manager ad-hoc code in arch/arm contains an unused
function 'reset_deassert_peripherals_handoff' that has been added from
the beginning. As this is probably a result of copying the gen5 reset
manager and this function has never been used, remove it.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>