Delete deprecated build flag for link-time injection of field trials.

Such feature was removed, flag has no effect.

Bug: webrtc:42220378
Change-Id: I0978df57dec9f511b63534030265e8826b29a278
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/404620
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Auto-Submit: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45351}
diff --git a/webrtc.gni b/webrtc.gni
index 3cf3cc5..fc85600 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -69,10 +69,6 @@
   # annotated symbols.
   rtc_enable_objc_symbol_export = rtc_enable_symbol_export
 
-  # Deprecated, this flag has no effect.
-  # TODO: bugs.webrtc.org/42220378 - Delete after August 15, 2025.
-  rtc_exclude_field_trial_default = false
-
   # Setting this to true will define WEBRTC_EXCLUDE_METRICS_DEFAULT which
   # will tell the pre-processor to remove the default definition of symbols
   # needed to use metrics. In that case a new implementation needs to be