tests: remove gcov-files- variables

tests: remove gcov-files- variables

Commit 31d2dda ("build-system: remove per-test GCOV reporting", 2018-06-20)
removed users of the variables, since those uses can be replaced by a simple
overall report produced by gcovr.  However, the variables were never removed.
Do it now.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
[thuth: Fixed up contextual conflicts with the patch from Eric]
Signed-off-by: Thomas Huth <thuth@redhat.com>