gnulib: keep a single gnulib in the repo
This reduces the complexity in the library by removing the two copies of gnulib.
Checklist
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Code modified for feature
Reviewer's checklist:
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There is a test suite reasonably covering new functionality or modifications -
Function naming, parameters, return values, types, etc., are consistent with other code -
This feature/change has adequate documentation added -
No obvious mistakes in the code