[fb] Remove extraneous m_lines from scrollback

We have one big array of characters in scrollback, with lines of
constant width -- there's no reason to have an array of pointers when
offsets will do.

src/drivers/fb/scrollback.h

@@ -17,7 +17,6 @@ public:
 
 private:
 	char *m_data;
-	char **m_lines;
 	unsigned m_rows, m_cols;
 	unsigned m_start;
 	unsigned m_count;