[libc++][ci] Removes C++2b selection option. (#88557)

Since Clang 16 is no longer supported all compilers support C++23.

Author: mordante

libcxx/utils/libcxx/test/params.py

@@ -83,12 +83,6 @@
 def getStdFlag(cfg, std):
     if hasCompileFlag(cfg, "-std=" + std):
         return "-std=" + std
-    # TODO(LLVM-19) Remove the fallbacks needed for Clang 16.
-    fallbacks = {
-        "c++23": "c++2b",
-    }
-    if std in fallbacks and hasCompileFlag(cfg, "-std=" + fallbacks[std]):
-        return "-std=" + fallbacks[std]
     return None