cc_cubic: remove redundant code

[cc]

Summary

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.