This unfortunately isn't possible... because of the potential for "same name" (particularly in old implementations where people would use both a prop and an eVar to track the same info with the same name for in Reporting correlations) Adobe will automatically append the actual dimension identifier.
As in, if your eVar was actually still called "Page URL (prop45)" the it would come through in the export as "Page URL (prop45) (prop45)"
So while you can't actually get rid of the dimension name, maybe if you explain how you are using the file, and I can help you with a solution once the file arrives? Generally the Data Warehouse export isn't used as the "final" report, but is processed into something else... is having "(prop45)" in the header of the raw data going to impact your process that much? (genuine question, not snark - I really want to understand your flow so that we can find a solution together, since there is no out of the box option)