Revert "exec.c: Fix breakpoint invalidation race"

Now that we have proper locking after MTTCG patches have landed, we
can revert the commit.  This reverts commit

a9353fe897ca2687e5b3385ed39e3db3927a90e0.

CC: Peter Maydell <peter.maydell@linaro.org>
CC: Alex BennÃ©e <alex.bennee@linaro.org>
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Message-Id: <20170712215143.19594-1-bobby.prani@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
