memory-internal.h: Remove obsolete claim that header is obsolete

The memory-internal.h header claims that it is for "obsolete
exec.c functions" which "will be removed soon". This statement
was added in 2011, six years ago, but the header is still here.
(Admittedly none of the prototypes added in commit 67d95c15
are still in the header.)

It's convenient to have a place to put prototypes for functions
which are used internally to the various .c files of the memory
system or by the accel/tcg code, which is inevitably fairly
closely coupled. So keep the header but update the comments to
reflect what we're actually using it for.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1511276888-17834-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>