Remove differences in how figure is generated for show=True vs. show=False

Removed unnecessary lines of code that cause unexpected error when show=False and retfig=True are used. The returned figure is not managed by pyplot and is thus unexpectedly difficult to deal with (for example, if fig=pydarn.plotting.rti.plotRti(args, show=False, retfig=True) then fig.show() returns an error). Removing these lines of code fixes this inconsistency.