Remove unused top-level readme file

As explained in the commit message of c595384d ("doc: Copy readme
content to docs/index.rst"), the docs/index.rst file is the new landing
page/entry point for reading the docs. The top-level readme file can be
removed, now that the docs layout have been restructured.

Change-Id: I591e1800500878722c6dbc6be1af704290f82cf3
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>