Revert "[PA] Enable PartitionAllocExternalMetadata on canary"

This reverts commit dcc8d784780e322ecf03d75202715d8c407f0832.

Reason for revert: revert PartitionAllocExternalMetadata.
The crrev.com/v/6839319 was landed to evaluate the feature for 1 day.

Bug: crbug.com/435341877
Original change's description:
> [PA] Enable PartitionAllocExternalMetadata on canary
>
> Enable the feature to see how the feature affects chrome statiblity
> and performance. This CL will be reverted in 1 day (not launch).
>
> Bug: crbug.com/435341877
> Change-Id: I3ed9a264a2665bdc3e85cd5451245d532ee0e7e9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6839319
> Reviewed-by: Yuki Shiino <[email protected]>
> Commit-Queue: Takashi Sakamoto <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#1499967}

Bug: crbug.com/435341877
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Ibf1b4ebee73609669dc0de2d50dad0fa061bd308
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6838520
Bot-Commit: Rubber Stamper <[email protected]>
Commit-Queue: Quyen Le <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1500045}
NOKEYCHECK=True
GitOrigin-RevId: 9d7276a2015fb5f1d68a90a2cd98505c298c9ed6

Author: tasak

partition_alloc.gni

@@ -340,11 +340,12 @@ declare_args() {
 declare_args() {
   # Because of address space issue, this feature cannot be enabled on
   # 32bit OS and iOS, i.e. has_64_bit_pointers && !is_ios
-  move_metadata_outside_gigacage = has_64_bit_pointers && !is_ios
+  move_metadata_outside_gigacage = false && has_64_bit_pointers && !is_ios
 }
 
 declare_args() {
-  enable_move_metadata_outside_gigacage_trial = move_metadata_outside_gigacage
+  enable_move_metadata_outside_gigacage_trial =
+      false && move_metadata_outside_gigacage
 }
 
 assert(!enable_move_metadata_outside_gigacage_trial ||