FreqWindow: Document fields, remove dead field

Signed-off-by: nyanpasu64 <nyanpasu64@tuta.io>

src/FreqWindow.h

@@ -141,10 +141,12 @@ private:
    Floats mData;
    size_t mWindowSize;
 
+   /// Whether x axis is in log-frequency.
    bool mLogAxis;
+   /// The minimum y value to plot.
    float mYMin;
+   /// The maximum y value to plot.
    float mYMax;
-   float mYStep;
 
    std::unique_ptr<wxBitmap> mBitmap;