PartitionAlloc: Do not assert C++20.

The static assert was a remanent of ASSERT_CPP_20 buildflag.

We don't want to assert C++20, because some compilers that do support
most of C++20 like GCC10 are still reporting 201709 despite the
--std=c++20.

See [godbolt](https://godbolt.org/z/ozoT4vEdr)
See [Kaylee discussion](https://skia-review.googlesource.com/c/skia/+/874976/comment/ac2baac2_1de3ecba/)

Bug: 351867706
Change-Id: Idce336ad91be83f6c1c054b51d693b7475853b02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7526908
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1576688}
NOKEYCHECK=True
GitOrigin-RevId: 8794d8f92f1fd326a998e9cd1f02fb3774f1b53f