Remove unused but set variable in cxgb_sge

Clang 15 warns:

sys/dev/cxgb/cxgb_sge.c:1290:21: error: variable 'txsd' set but not used [-Werror,-Wunused-but-set-variable]
        struct tx_sw_desc *txsd = &txq->sdesc[txqs->pidx];
                           ^

It appears 'txsd' is a leftover from a previous refactoring (see
3f345a5d09b6), but is no longer used for anything, and can be removed
without any functional change.

MFC after: 3 days
Reviewed by: np
Differential Revision: https://reviews.freebsd.org/D35833