PCI: dwc: artpec6: Remove unused defines

Commit b015b37e ("PCI: artpec6: Stop enabling writes to
DBI read-only registers") removed the only write using these
defines, but it did not remove the defines.
Remove the defines since they are now unused.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>