PCI: dwc/intel-gw: Remove unused function

PCI: dwc/intel-gw: Remove unused function

Fix the following clang warning:

drivers/pci/controller/dwc/pcie-intel-gw.c:84:19: warning: unused
function 'pcie_app_rd' [-Wunused-function].

Link: https://lore.kernel.org/r/1618475577-99198-1-git-send-email-jiapeng.chong@linux.alibaba.com
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Krzysztof Wilczyński <kw@linux.com>