Touchup related to C++20

- Update the documentation
- Remove the `pdf_use_cxx20` variable

Bug: pdfium:1932
Change-Id: Ib1a92226ec7c694475771bc93468f34d35447eb0
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/113070
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/README.md b/README.md
index d867e4c..10cef8b 100644
--- a/README.md
+++ b/README.md
@@ -93,7 +93,7 @@
 For sample applications like `pdfium_test` to build, one must set
 `pdf_is_standalone = true`.
 
-By default, the entire project builds with C++17.
+By default, the entire project builds with C++20.
 
 By default, PDFium expects to build with a clang compiler that provides
 additional chrome plugins. To build against a vanilla one lacking these,
diff --git a/pdfium.gni b/pdfium.gni
index 6692dd7..ae5c5a5 100644
--- a/pdfium.gni
+++ b/pdfium.gni
@@ -36,11 +36,6 @@
   # If XFA, also support tiff codec. Ignored if not XFA.
   pdf_enable_xfa_tiff = true
 
-  # Build PDFium using C++20 if set to true. Otherwise builds with C++17.
-  # There is no "pdf_use_cxx20_override" variable because this is only used in
-  # standalone PDFium, and not when PDFium is embedded in our projects.
-  pdf_use_cxx20 = true
-
   # Build PDFium against PartitionAlloc. When false, PDFium must build without
   # requiring any PartitionAlloc headers or code to be present. When true,
   # PDFium will use PartitionAlloc partitions to separate strings, scalars,