TCP RACK: remove variable with is only initialized and not changed

minslot is initialized to 0 and never changed. It is not clear to me
under which condition minslot should be set to which value.
Therefore, remove it and the code checking that it is not zero.
No functional change intended.

Reviewed by: rrs
CID: 1523812
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48321