ira: Remove duplicate `memset' over `full_costs' from `assign_hard_reg'

Remove duplicate clearing of `full_costs' made in `assign_hard_reg',
which has been there since the beginning, i.e. commit 058e97ec
("IRA has been merged into trunk"),
<https://gcc.gnu.org/ml/gcc-patches/2008-08/msg01932.html>.

	gcc/
	* ira-color.cc (assign_hard_reg): Remove duplicate `memset' over
	`full_costs'.