target/ppc: Remove DisasContext.exception

Now that we have removed all of the fake exceptions, and all real
exceptions exit via DISAS_NORETURN, we can remove this field.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20210517205025.3777947-5-matheus.ferst@eldorado.org.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>