Remove obsolete README file.

Perhaps we ought to add some other kind of documentation here instead,
but for now let's get rid of this woefully obsolete description of the
sinval machinery.

src/backend/storage/ipc/README

-src/backend/storage/ipc/README
-
-Cache Invalidation Synchronization Routines
-===========================================
-
-Mon Jul 18 11:09:22 PDT 1988  W.KLAS
-
-The cache synchronization is done using a message queue. Every
-backend can register a message which then has to be read by
-all backends. A message read by all backends is removed from the
-queue automatically. If a message has been lost because the buffer
-was full, all backends that haven't read this message will be
-told that they have to reset their cache state. This is done
-at the time when they try to read the message queue.
-
-The message queue is implemented as a shared buffer segment. Actually,
-the queue is a circle to allow fast inserting, reading (invalidate data) and
-maintaining the buffer.