Since f5ef0e51, we have a real page mapped for kuser,
which means the special casing for SIGSEGV can go away.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220421151735.31996-11-richard.henderson@linaro.org>