configure: Remove --enable-test-coverage

configure: Remove --enable-test-coverage

We provide a custom configure option --enable-test-coverage and
'make cov' target to generate code coverage reports. However gnulib
already provides a 'make coverage' which 'just works' and doesn't
require a special configure option.

This drops our custom implementation in favor of 'make coverage'.
Reports are now output to cov/index.html

Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>