Revert "[x64] Enable userspace PCI in non-chromebook x64 builds"

This reverts commit db5437a031a54ce787d8b9b05a3d695cba5a56fe.

Reason for revert: Breaks for NUC8i5BEK model NUCs.

Original change's description:
> [x64] Enable userspace PCI in non-chromebook x64 builds
>
> This CL enables the flag to switch from the Kernel PCI driver in
> //zircon to the userspace PCI driver in //src for all non-chromebook x64
> builds. No new interrupt modes or support is being introduced in this
> phase of the migration.
>
> Bug: 32978, 80426
> Change-Id: I0cd7965b2d4f4dea05204a5336a38c04a16bb088
> Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/549621
> Reviewed-by: Suraj Malhotra <surajmalhotra@google.com>
> Reviewed-by: David Greenaway <dgreenaway@google.com>
> Reviewed-by: Alex Legg <alexlegg@google.com>
> Commit-Queue: Christopher Anderson <cja@google.com>

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 32978, 80426
Change-Id: Ia4fb79f62d4cd2fbfb133bdabef3eacd9b35c510
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/560682
Reviewed-by: Alex Legg <alexlegg@google.com>
Reviewed-by: David Greenaway <dgreenaway@google.com>
Commit-Queue: Alex Legg <alexlegg@google.com>
diff --git a/boards/chromebook-x64.gni b/boards/chromebook-x64.gni
index 834c418..f36eefd 100644
--- a/boards/chromebook-x64.gni
+++ b/boards/chromebook-x64.gni
@@ -4,11 +4,6 @@
 
 import("common/x64-common.gni")
 
-# userspace PCI migration flags (Chromebook needs more testing)
-# fxbug.dev/32978
-platform_enable_user_pci = false
-userspace_pci_enable_msi_x = false
-
 target_cpu = "x64"
 
 board_name = "chromebook-x64"
diff --git a/boards/common/x64-common.gni b/boards/common/x64-common.gni
index 7452b45..4412099 100644
--- a/boards/common/x64-common.gni
+++ b/boards/common/x64-common.gni
@@ -77,8 +77,3 @@
 ]
 
 minfs_enable_dirty_cache = true
-
-# userspace PCI migration flags
-# fxbug.dev/32978
-platform_enable_user_pci = true
-userspace_pci_enable_msi_x = false