qemu-img-cmds.hx: Remove texinfo document fragments

Now the qemu-img documentation has been converted to rST, we can
remove the texinfo document fragments from qemu-img-cmds.hx, as
they are no longer used.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200124162606.8787-7-peter.maydell@linaro.org