Restore split_ncvars.pl splitting behavior post NCO 5.1.9

[ceblanton]

Description
NCO tools 5.1.9 changed the behavior of ncks to include more referenced variables in the split-out files. These extra variables turn out to be undesired FRE users, so this reverts to the previous behavior.

Fixes #372

How Has This Been Tested?
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Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes

[ceblanton]

@mlee03 I'm sure we want this but I'll admit I didn't make a compelling case yet