ARC: Remove unused DECLARE_GLOBAL_DATA_PTR from init_helpers.c

"Global data" structure "gd" is not used in init_helpers.c
thus DECLARE_GLOBAL_DATA_PTR might be safely removed.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>