(full_write): Remove unused decl.

(full_write): Remove unused decl.
(open_maybe_create): Remove.
(touch): Don't record whether the file was created; this isn't
possible to do reliably and portably without race conditions.
If the file was created and if amtime_now is nonzero, this
change means we'll have to do another utime system call, but
that's no more harmful than the previous version of this code.