Multiline: just remember last num of rows, not max.

For some reason, the old code was always cleaning the maximum number of
rows used so far while editing in multi line mode. Actually we need to
clean just the number of rows used by the last line. The old behavior
created problems in multiplexing mode, where the line is refreshed at a
different row, if the user used linenoiseHide() / show() in order to
print something. With the new behavior, all looks fine, so far.