[chisel5] Remove FIRRTL notes

Remove old "notes" directory from FIRRTL.  This includes a lot of great
brainstorming and feedback from early users of FIRRTL.  It doesn't need
to stay in the repo, however.  Fortunately, git will remember...

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>