fix several -Woverloaded-virtual warnings in common/* dir

- CACHED_CONTAINER::GetVertices: removed useless and unused function
   which does nothing but tainted the API
 - SHAPE_LINE_CHAIN::Collide( const BOX2I& aBox, int aClearance ):
   removed an outdated function with assert(false)
 - SHAPE::BBox(): fix a small typo in the doc string

common/gal/opengl/cached_container.cpp

@@ -203,14 +203,6 @@ void CACHED_CONTAINER::Clear()
 }
 
 
-VERTEX* CACHED_CONTAINER::GetVertices( const VERTEX_ITEM* aItem ) const
-{
-    int offset = aItem->GetOffset();
-
-    return &m_vertices[offset];
-}
-
-
 unsigned int CACHED_CONTAINER::reallocate( unsigned int aSize )
 {
     wxASSERT( aSize > 0 );

common/geometry/shape_line_chain.cpp

@@ -35,13 +35,6 @@ bool SHAPE_LINE_CHAIN::Collide( const VECTOR2I& aP, int aClearance ) const
 }
 
 
-bool SHAPE_LINE_CHAIN::Collide( const BOX2I& aBox, int aClearance ) const
-{
-    assert( false );
-    return false;
-}
-
-
 bool SHAPE_LINE_CHAIN::Collide( const SEG& aSeg, int aClearance ) const
 {
     BOX2I box_a( aSeg.A, aSeg.B - aSeg.A );

include/gal/opengl/cached_container.h

@@ -64,14 +64,6 @@ public:
     ///> @copydoc VERTEX_CONTAINER::Clear()
     virtual void Clear();
 
-    /**
-     * Function GetVertices()
-     * returns the vertices stored by the specific item.
-     *
-     * @param aItem is the item.
-     */
-    virtual VERTEX* GetVertices( const VERTEX_ITEM* aItem ) const;
-
 protected:
     ///> Maps size of free memory chunks to their offsets
     typedef std::pair<unsigned int, unsigned int> CHUNK;

include/geometry/shape.h

@@ -128,7 +128,7 @@ public:
     virtual bool Collide( const SEG& aSeg, int aClearance = 0 ) const = 0;
 
     /**
-     * Function Collide()
+     * Function BBox()
      *
      * Computes a bounding box of the shape, with a margin of aClearance
      * a collision.

include/geometry/shape_line_chain.h

@@ -277,16 +277,6 @@ public:
      */
     bool Collide( const VECTOR2I& aP, int aClearance = 0 ) const;
 
-    /**
-     * Function Collide()
-     *
-     * Checks if box aBox lies closer to us than aClearance.
-     * @param aBox the box to check for collisions with
-     * @param aClearance minimum distance that does not qualify as a collision.
-     * @return true, when a collision has been found
-     */
-    bool Collide( const BOX2I& aBox, int aClearance = 0 ) const;
-
     /**
      * Function Collide()
      *