PCI/AER: Remove duplicate PCI_EXP_AER_FLAGS definition

PCI_EXP_AER_FLAGS was defined twice (with identical definitions), once
under #ifdef CONFIG_ACPI_APEI, and again at the top level.  This looks like
my merge error from these commits:

  fd3362cb ("PCI/AER: Squash aerdrv_core.c into aerdrv.c")
  41cbc9eb ("PCI/AER: Squash ecrc.c into aerdrv.c")

Remove the duplicate PCI_EXP_AER_FLAGS definition.

Fixes: 41cbc9eb ("PCI/AER: Squash ecrc.c into aerdrv.c")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Oza Pawandeep <poza@codeaurora.org>