Remove usage of "use_ios_simulator".

This GN variable has been removed by https://chromium-review.googlesource.com/c/chromium/src/+/2476278,
in favour of directly using "target_environment".

Change-Id: I4cfdea8faf3b4dc1e4568c76058520fe1cd52845
Bug: webrtc:12059
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/189763
Commit-Queue: Lahiru Ginnaliya Gamathige <glahiru@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32448}
diff --git a/sdk/BUILD.gn b/sdk/BUILD.gn
index ddd8431..ab6dc50 100644
--- a/sdk/BUILD.gn
+++ b/sdk/BUILD.gn
@@ -1097,7 +1097,7 @@
 
           # TODO(peterhanspers): Reenable these tests on simulator.
           # See bugs.webrtc.org/7812
-          if (!use_ios_simulator) {
+          if (target_environment != "simulator") {
             sources += [
               "objc/unittests/RTCAudioDeviceModule_xctest.mm",
               "objc/unittests/RTCAudioDevice_xctest.mm",