cc_cubic: remove redundant code

During my progress on updating cc_cubic to RFC9438, found such redundancy as:

• W_est: we use the alternative stack local variable W_est in

	 `cubic_ack_received()`.

• cwnd_prior: it is used for Reno-Friendly Region in RFC9438 Section 4.3,

	 but we use the alternative cwnd from NewReno for Reno-Friendly as
	 in commit ee45061051715be4704ba22d2fcd1c373e29079d.

No functional change intended.

Reviewed by: rscheff, tuexen
Differential Revision: https://reviews.freebsd.org/D49008