exec.c: Remove out of date comment

exec.c: Remove out of date comment

Remove an out of date comment: this comment used to be attached to
cpu_register_physical_memory_log(), before commit 0f0cb164 accidentally
inserted a couple of other functions between the comment and its function.
It is in any case obsolete since (a) the function arguments it refers
to have been replaced with a single MemoryRegionSection* argument and
(b) the inability to handle regions whose offset_within_address_space
and offset_within_region aren't equally aligned was fixed as part of
the rewrite of this code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>