Have a cleaner way to remove the massif preload from LD_PRELOAD.

Have a cleaner way to remove the massif preload from LD_PRELOAD.

The previous code was removing the massif preload (when --pages-as-heap=yes)
by replacing the entry with spaces.
This is not very clear, and I suspect this gives problems with the
android linker, which seems to use such a space entry as a real entry
to load (and then fails to start the application).

This patch really removes the entry, by shifting the characters.

Tested on amd64/debian.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16306