arithmetic: rm useless `unused_mut` #3093

Job	Run time
Check (stable)	6s
Check (nightly)	1m 0s
Check Documentation	1m 3s
List directories for parallelizing tests	3s
Check Style	7s
Test (stable)	8m 53s
Test (nightly)	17m 52s
Check no_std	27s
Test assembly	8m 23s
Test (cp6_782/)	5s
Test (mnt4_753/)	19m 10s
Test (mnt6_753/)	9s
Test (./ed_on_mnt4_753)	3m 53s
Test (./secp256r1)	56s
Test (./ed_on_mnt4_298)	1m 9s
Test (./ed_on_cp6_782)	1m 17s
Test (./mnt4_298)	3m 43s
Test (./mnt6_298)	5m 19s
Test (./ed_on_bn254)	1m 2s
Test (./bn254)	2m 16s
Test (./secp256k1)	1m 3s
Test (./bw6_761)	3m 55s
Test (./bls12_377)	4m 9s
Test (./grumpkin)	1m 6s
Test (./secp384r1)	1m 24s
Test (./ed_on_bls12_377)	1m 7s
Test (./secq256k1)	1m 5s
Test (./ed25519)	55s
Test (./ed_on_bls12_381)	1m 14s
Test (./ed_on_bw6_761)	35s
Test (./curve25519)	1m 6s
Test (./ed_on_bls12_381_bandersnatch)	1m 7s
Test (./pallas)	1m 4s
Test (./vesta)	1m 13s
Test (./bw6_767)	3m 58s
Test (./bls12_381)	3m 19s
	1h 45m 13s

Provide feedback