[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}