Remove definition of WEBRTC_MOZILLA_BUILD.

This macro is not used in WebRTC code.

Bug: None
Change-Id: I5af1299594e8644ce2a9c772e29507367fd7440d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/250140
Auto-Submit: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35872}

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index eb06453..e8c4dfe 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -128,10 +128,6 @@
     defines += [ "WEBRTC_ENABLE_OBJC_SYMBOL_EXPORT" ]
   }
 
-  if (build_with_mozilla) {
-    defines += [ "WEBRTC_MOZILLA_BUILD" ]
-  }
-
   if (!rtc_builtin_ssl_root_certificates) {
     defines += [ "WEBRTC_EXCLUDE_BUILT_IN_SSL_ROOT_CERTS" ]
   }