You can. As You probably have done already, you can use your implicit (manually-created) context variables inside your db components (each instance, manually). This is absolutely admitted. As a matter of facts, automatic context groups for databases have been added in Talend 4.1. so they are a relatively new feature.
The question is: is it really what you need/want? You'll lose the ability to use the metadata repository connections, test and check your parameters, revert and update the variables automatically and apply to every component with one click. Just to not have a separated context group.
If you consider that context groups scoped just inside the design view, as when the job is exported every context var will fit in a single .properties file, regardingless the original group, I would think again about your constrain...