Remove onTypeConflict from public api

[brennantaylor]

As noted in this thread #863

The following change removed the usage of the parameter: https://github.com/apollographql/graphql-tools/pull/918/files#diff-bb87a5c1cf8e770e2b377a7e61904637L38

Would be good to remove from the public api so people don't think it actually does anything anymore.

[nether-cat]

Please bring back onTypeConflict

As also noted in that thread multiple times, there are scenarios in which this feature seemingly cannot be substituted with schema transforms. How could I merge the values of two enum types for example?

[januszhou]

Agreed, since onTypeConflict has been removed, it will at least less confused.

[brennantaylor]

@nether-cat You would query the individual schemas and reconcile it before calling mergeSchemas. It is possible with schema transforms.

We were able to replace all our type conflict resolution with our own programmed with schema transforms. It's just not as friendly.

What would have been nice is instead of removing the functionality if they wanted mergeSchemas to be more pure they could have lifted the type conflict stuff to a new function instead.

[yaacovCR]

Brought back, rolled into #1306