xreadlink-with-size: Remove module. No longer used.

xreadlink-with-size: Remove module.  No longer used.
Ex-callers now use xreadlink or mreadlink-with-size.
* modules/xreadlink-with-size: Remove module.
* lib/xreadlink-with-size.c: Remove file.
* lib/xreadlink.h (xreadlink_with_size): Remove prototype.
(xreadlink): Remove inaccurate comment.  The one in xreadlink.c,
just before the function definition *is* accurate.