PCI/AER: Remove aerdriver.forceload kernel parameter

Per the PCI Firmware spec, r3.0, sec 4.5.1, on ACPI systems, the OS must
not use AER unless _OSC is present and _OSC grants AER control to the OS.
The aerdriver.forceload kernel parameter was a way to enable Linux AER
support on ACPI systems that lack _OSC or fail to grant control the the OS.

Enabling Linux AER support when the firmware doesn't want us to is a recipe
for problems, e.g., the firmware might be handling AER itself.

Remove the aerdriver.forceload kernel parameter and related supporting
code.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>