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GH-34640: [R] Can't read in partitioning column in CSV datasets when both (non-hive) partition and schema supplied (#37658)

### Rationale for this change

It wasn't possible to use the partitioning column in the dataset when reading in CSV datasets and supplying both a schema and a partition variable.

### What changes are included in this PR?

This PR updates the code which creates the `CSVReadOptions` object and makes sure we don't pass in the partition variable column name as a column name there, as previously this was resulting in an error.

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes 

* Closes: #34640

Lead-authored-by: Nic Crane <thisisnic@gmail.com>
Co-authored-by: Dewey Dunnington <dewey@dunnington.ca>
Signed-off-by: Nic Crane <thisisnic@gmail.com>