refactor: remove core crate from datafusion-proto (#18123)
## Which issue does this PR close?
- Closes #17713.
## Rationale for this change
Now that we have all the required supporting code moved out of the
`core` crate, we can remove this as a dependency to reduce build times
for downstream projects.
## What changes are included in this PR?
Remove dependency.
Update paths.
## Are these changes tested?
Existing unit tests since this is just a code shuffle.
## Are there any user-facing changes?
- Methods within the proto crate now take `TaskContext` instead of
`SessionContext`
