Add button to remove fit curves

[hschovanec-usgs]

Once fit curves are added, there is no option to remove them. Fit curves can be removed using Restore data on the Stream page, but all changes (flags) are lost unless saved in a file.
Fit method: https://github.com/usgs/magpy/blob/master/magpy/gui/magpy_gui.py#L2173-L2216
Fit dialog box class: https://github.com/usgs/magpy/blob/master/magpy/gui/dialogclasses.py#L1805-L1873

OS: Sierra
MagPy: v0.3.91

[hschovanec-usgs]

Adding an option in the fit dialog box's function combo box rather than a button may be a simpler fix.