target/arm: Remove fprintf from disas_simd_mod_imm

The default of this switch is truly unreachable.
The switch selector is 3 bits, and all 8 cases are present.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210604183506.916654-3-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>