Option --enable-extra-example-tests can be removed, extra tests can be always enabled

[edhartnett]

In the examples/F77 directory we run our classic tests, and, if netCDF-4 is built into the C library, we add the netCDF-4 F77 tests.

In the F90 directory we do the same, but also require that --enable-extra-example-tests be specified in configure, which surely no one is noticing or bothering with.

There is no reason not to run the F90 netCDF-4 examples, just as we run the F77 netCDF-4 examples, whenever netCDF-4 is present. We can remove the --enable-extra-example-tests to simplify our configure script and our CI testing.

I will take care of this in an upcoming PR.