Revision 51357

handbook: remove instructions for NPAPI plugins

- Neither Firefox nor Chrome support NPAPI plugins anymore and as such
  the instructions to install them are not applicable.
- Tell users of opera to install the plugin directly rather than
  redirect to Firefox's instructions. I have not validated that this
  still works. Google shows mixed information for Opera's NPAPI support.
- While here remove an extraneous word 'new'

PR:		220399
Reported by:	Randy Westlund <rwestlun@gmail.com>