PCI: Remove unused PCI_MSIX_FLAGS_BIRMASK definition

PCI_MSIX_FLAGS_BIRMASK has been replaced by PCI_MSIX_TABLE_BIR for better
readability.  Now no one uses it, remove it.  No functional change.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>