I noticed this opportunity for simplification while drafting a
new, related module that I haven’t had time to finish yet.
* m4/close-stream.m4, m4/closein.m4, m4/closeout.m4: Remove.
* modules/close-stream (Files): Remove m4/close-stream.m4.
(configure.ac): Omit gl_CLOSE_STREAM.
* modules/closein (Files): Remove m4/closein.m4
(configure.ac): Omit gl_CLOSEIN.
* modules/closeout (Files): Remove m4/closeout.m4.
(configure.ac): Omit gl_CLOSEOUT.