Remove unused attr_reader definition of cell width

Hi. `width` used to be defined twice in `Teminal::Table::Cell`, and since the second definition overwrote the first definition (`attr_reader :width`), the first definition was never actually used and just triggered a warning when running in verbose mode:
  `~/.gem/ruby/2.0.0/gems/terminal-table-1.4.5/lib/terminal-table/cell.rb:77: warning: method redefined; discarding old width`