RISC-V: Fix one unused varable in riscv_subset_list::parse

RISC-V: Fix one unused varable in riscv_subset_list::parse

This patch would like to fix one unused variable as below:

../../gcc/common/config/riscv/riscv-common.cc: In static member function
'static riscv_subset_list* riscv_subset_list::parse(const char*, location_t)':
../../gcc/common/config/riscv/riscv-common.cc:1501:19: error: unused variable 'itr'
  [-Werror=unused-variable]
 1501 |   riscv_subset_t *itr;

The variable consume code was removed but missed the var itself in
previous.  Thus, we have unused variable here.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_subset_list::parse):
	Remove unused var decl.

Signed-off-by: Pan Li <pan2.li@intel.com>