sched_4bsd: Remove RQ_PPQ from ESTCPULIM()'s formula

Substracting RQ_PPQ to the maximum number of allowed priority values
(the factor to INVERSE_ESTCPU_WEIGHT) has the effect of pessimizing the
number of processes assigned to the last priority bucket.

MFC after: 1 month
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45392