Delete trailing whitespace between START and END.
If called interactively, START and END are the start/end of the
region if the mark is active, or of the buffer's accessible
portion if the mark is inactive.
This command deletes whitespace characters after the last
non-whitespace character in each line between START and END. It
does not consider formfeed characters to be whitespace.
If this command acts on the entire buffer (i.e. if called
interactively with the mark inactive, or called from Lisp with
END nil), it also deletes all trailing lines at the end of the
buffer if the variable `delete-trailing-lines' is non-nil.