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I misunderstood how the external-arm-toolchain recipes were working, the
latest revision of the recipe works with both 10.3 and 11.2.

Clean up my mess by dropping the PREFERRED_VERSION from the CI, and revert
the addition of versioned recipes.  Simply using the right tarball is
sufficient.

Thanks to Sumit Garg for noticing my mistake.
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>