Revert "[check] gn gen --check -> gn gen --check=system"

This reverts commit 08f0b15d72c8c5479ede74ab671bbea105ca5750.

Reason for revert:

Oops, I missed some --check=system in bringup that the recipe side is failing on, so I'm going to revert this until I fix those because otherwise people won't be able to fx set some bringup configs in the meantime.

Original change's description:
> [check] gn gen --check -> gn gen --check=system
> 
> Be stricter.
> See: `gn help check`
> See: https://fuchsia-review.googlesource.com/c/infra/recipes/+/404874
> 
> Bug: 55166
> 
> Change-Id: I0cb95f565a05cc3c29127b767b32d6565e0b2bab
> Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/404809
> Commit-Queue: Shai Barack <shayba@google.com>
> Reviewed-by: Scott Graham <scottmg@google.com>
> Reviewed-by: P.Y. Laligand <pylaligand@google.com>
> Testability-Review: Scott Graham <scottmg@google.com>

TBR=pylaligand@google.com,scottmg@google.com,shayba@google.com,nmulcahey@google.com

Change-Id: Ica62b8361531ed9c5259acd404ab84a3b89fdf7f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 55166
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/405516
Reviewed-by: Scott Graham <scottmg@google.com>
Commit-Queue: Scott Graham <scottmg@google.com>
diff --git a/tools/devshell/set b/tools/devshell/set
index abb4397..41686f5 100755
--- a/tools/devshell/set
+++ b/tools/devshell/set
@@ -29,7 +29,7 @@
 ##
 ## Ensures Goma is ready (if Goma is enabled).
 ##
-## This is a wrapper around running `gn gen --check=system BUILDDIR --args ...`.
+## This is a wrapper around running `gn gen --check BUILDDIR --args ...`.
 ## If GN fails with an error, `fx set` does not change anything.
 ##
 ## optional arguments:
@@ -179,7 +179,7 @@
   shift
 
   local gn_cmd='gen'
-  local -a gn_switches=(--check=system --export-rust-project --export-compile-commands=default)
+  local -a gn_switches=(--check --export-rust-project --export-compile-commands=default)
   local gn_after_args=""
   local base=()
   local universe=()