toolchain-external: remove useless info in help text of Linaro ARM toolchains

The information "To use this toolchain, you must disable soft float
usage." which was visible in the help text of Linaro toolchains is no
longer useful, since those toolchains are only visible when the ARM
EABIhf ABI is selected, which by design is not compatible with
soft-float.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>