target/arm: Fix decode of fp16 vector fabs, fneg, fsqrt

target/arm: Fix decode of fp16 vector fabs, fneg, fsqrt

These opcodes are only supported as vector operations,
not as advsimd scalar.  Set only_in_vector, and remove
the unreachable implementation of scalar fneg.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20241211163036.2297116-24-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>