doc: remove RELEASE_NOTES

It hasn't been used since 0.11, and it does not contain any useful
information except for the code name.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/RELEASE_NOTES

-Release Notes
-=============
-
-* 2.2 "Muybridge"  March, 2014
-
-
-General notes
--------------
-See the Changelog file for a list of significant changes. Note, there
-are many more new features and bugfixes than whats listed there.
-
-Bugreports against FFmpeg git master or the most recent FFmpeg release are
-accepted. If you are experiencing issues with any formally released version of
-FFmpeg, please try git master to check if the issue still exists. If it does,
-make your report against the development code following the usual bug reporting
-guidelines.