Revert "Use INSERT for importing tables, rather than COPY"

This reverts commit bc67cc6d75944f16cc3d2be1911e2ba24f307a76.

There was no difference between INSERT and COPY in terms of
explicitly mentioned field names. Both don't play nicely
with DDL triggers as of GDAL-1.11.0 (will be fixed in later
versions). So we get back to COPY for better performance.