rm useless generics for `DensePolynomial` #3107

Job	Run time
Check no_std	26s
List directories for parallelizing tests	4s
Check (stable)	1m 2s
Check (nightly)	59s
Check Documentation	53s
Check Style	11s
Test (stable)	8m 56s
Test assembly	8m 24s
Test (nightly)	17m 38s
Test (cp6_782/)	28m 57s
Test (mnt4_753/)	13m 1s
Test (mnt6_753/)	26m 40s
Test (./ed_on_mnt4_753)	3m 58s
Test (./secp256r1)	1m 0s
Test (./ed_on_mnt4_298)	1m 5s
Test (./ed_on_cp6_782)	1m 15s
Test (./mnt4_298)	3m 49s
Test (./mnt6_298)	5m 26s
Test (./ed_on_bn254)	59s
Test (./bn254)	2m 18s
Test (./secp256k1)	1m 5s
Test (./bw6_761)	3m 54s
Test (./bls12_377)	4m 9s
Test (./grumpkin)	1m 7s
Test (./secp384r1)	1m 36s
Test (./ed_on_bls12_377)	1m 9s
Test (./secq256k1)	53s
Test (./ed25519)	54s
Test (./ed_on_bls12_381)	1m 21s
Test (./ed_on_bw6_761)	36s
Test (./curve25519)	1m 4s
Test (./ed_on_bls12_381_bandersnatch)	1m 1s
Test (./pallas)	1m 8s
Test (./vesta)	1m 7s
Test (./bw6_767)	4m 2s
Test (./bls12_381)	3m 13s
	2h 35m 20s

Provide feedback