toolchain: update paranoid unsafe path comment

Since 919c06c2 the calls to gcc always pass
through the toolchain wrapper and all gcc patches to support poisoning has been
removed.

Update the BR2_COMPILER_PARANOID_UNSAFE_PATH comment.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>