[Cleanup] Remove our own definition of M_PI.

* This is too brittle and might clash with MSVC's M_PI. See [1].
* We only used it once (in a unit test).
* We shouldn't use PI anyway [2].

Instead, pull it from <cmath> with _USE_MATH_DEFINES,
like it's already done in the code base.

[1] https://ci.chromium.org/p/webrtc/builders/try/win_x86_msvc_rel/6844
[2] https://tauday.com/tau-manifesto

Bug: webrtc:9855
Change-Id: I7a6976240604ef367ea07478d8cb5e4020e5dfeb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/132548
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Yves Gerey <yvesg@google.com>
Cr-Commit-Position: refs/heads/master@{#27597}

modules/audio_processing/rms_level_unittest.cc

@@ -7,6 +7,8 @@
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
+// MSVC++ requires this to be set before any other includes to get M_PI.
+#define _USE_MATH_DEFINES
 #include <cmath>
 #include <memory>
 #include <vector>
@@ -14,7 +16,6 @@
 #include "api/array_view.h"
 #include "modules/audio_processing/rms_level.h"
 #include "rtc_base/checks.h"
-#include "rtc_base/numerics/math_utils.h"
 #include "rtc_base/numerics/safe_conversions.h"
 #include "test/gtest.h"