Remove set-but-unused variable from s_sincosl.c

This look like a copy and paste leftover.

Reported by: enh@google.com (via freebsd-numerics@)
Reviewed by: Steve Kargl <sgk@troutmask.apl.washington.edu>

(cherry picked from commit 20d425842a4f80f9efc1de3f7a3b043435fe4b39)