.lgtm.yml: remove GCC 6 override
.lgtm.yml: remove GCC 6 override

This will let the project continue to work when the lgtm.com build
environment is upgraded to Ubuntu 19.10, which doesn't include GCC 6.
DIPlib/diplib,DIPlib/diplib,DIPlib/diplib,DIPlib/diplib,DIPlib/diplib,DIPlib/diplib