kern: remove deprecated makesyscalls.sh

makesyscalls was rewritten in Lua and introduced in d3276301ab. In the
time since, no objections have risen and a warning was introduced long
ago on invocation of makesyscalls.sh that it would be removed before
FreeBSD 13. Belatedly follow through on that.

(cherry picked from commit e3707726c168eeb0ba06d13873dd71590b77d604)