dt-bindings: PCI: remove designware-pcie.txt

Now that the properties defined there were converted to DT schema,
and the other dt-bindings are pointing to the new schemas,
drop it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/c93261b41f9ffe8d97d8c930f57b41aaf7de5264.1626608375.git.mchehab+huawei@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>