rmfit removes all the fits (and their equation box) created by
showfit from the current figure.
rmfit('first') removes only the first fit.
rmfit('last') removes only the last fit (equivalent to undofit).
rmfit(N) removes only the Nth fit.
rmfit('boxonly') removes only the equation boxes.
rmfit('getslope') removes all the equations displayed by getslope and
showslope.
Note that rmfit does not remove the annotation objects (eg, lines).