vasprintf: use from gnulib; don't bundle twice

Do not bundle a second vasprintf implementation; it is already in gnulib.

Relates #653 (closed)

Checklist

• Code modified for feature

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code