target/mips: Replace [g_]assert(0) -> g_assert_not_reached()

target/mips: Replace [g_]assert(0) -> g_assert_not_reached()

In order to avoid warnings such commit c0a6665c3c ("target/i386:
Remove compilation errors when -Werror=maybe-uninitialized"),
replace all assert(0) and g_assert(0) by g_assert_not_reached().

Remove any code following g_assert_not_reached().

See previous commit for rationale.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230221232520.14480-4-philmd@linaro.org>