exec.c: Provide full set of dummy wp remove functions in user-mode

exec.c: Provide full set of dummy wp remove functions in user-mode

We already provide dummy versions of the cpu_watchpoint_insert
and cpu_watchpoint_remove_all functions when CONFIG_USER_ONLY
is defined. Complete the set by providing cpu_watchpoint_remove
and cpu_watchpoint_remove_by_ref as well.

This allows target-* code using these functions to avoid
some ifdeffery.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>