tools/memory-model: Remove out-of-date SRCU documentation

Commit 6cd244c87428 ("tools/memory-model: Provide exact SRCU semantics")
changed the semantics of partially overlapping SRCU read-side critical
sections (among other things), making such documentation out-of-date.
The new, semantic changes are discussed in explanation.txt.  Remove the
out-of-date documentation.

Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>