Don't harcode mksquashfs options when airootfs_image_tool_options is not set or is empty

If airootfs_image_tool_options is set to () or not set at all in profiledef.sh, mkarchiso will use airootfs_image_tool_options=('-comp' 'xz').

This was motivated by my bright idea of backwards compatibility or something like that. The result is that

if (( ${#airootfs_image_tool_options[@]} < 1 )); then

prevents the usage of mksquashfs defaults.

Since both profiles explicitly set their airootfs_image_tool_options, I think the condition should be removed.

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