This is a first step towards replacing ARB by FLINT-3. Currently the code
still works (only) with FLINT<3 and ARB, but it assumes that ARB is present
as soon as FLINT is found.

* configure.ac: Essentially drop check for ARB.
* gp/paritwine-arb.gp: Merge into...
* gp/paritwine-flint.gp: ...this.
* gp/paritwine.gp.in: Do not include gp/paritwine-arb.gp.
* src/paritwine.h: Adapt to lack of test for ARB.
* tests/Makefile.am: Drop tarb.
* tests/tarb.c: Merge into...
* tests/tflint.c: ...this.
* NEWS: Document change.