Can we imagine future Halloweens without a new “Halloween”? We might have to, if David Gordon Green’s “Halloween Ends,” the wrap-up film of the reboot trilogy he began in 2018, plants a full stop on a 44-year-old franchise. Savvy viewers, though, will intuit the title’s missing question mark, understanding that Michael Myers, one of cinema’s fustiest boogeymen, is unlikely to remain interred for long.