[libc++][test] Removes old fallbacks.

[Mordante]

Summary

All supported compilers support the -std=year except Clang < 17 which
doesn't support C++23. This fallback is marked for removal once we no
longer support Clang 16.

libcxx/utils/libcxx/test/params.py

_warningFlags = [
     "-Wno-local-type-template-args",
     "-Wno-c++11-extensions",
 ]
 
 _allStandards = ["c++03", "c++11", "c++14", "c++17", "c++20", "c++23", "c++26"]
 
 
 def getStdFlag(cfg, std):
-    fallbacks = {
-        "c++11": "c++0x",
-        "c++14": "c++1y",
-        "c++17": "c++1z",
-        "c++20": "c++2a",
-        "c++23": "c++2b",
-    }
     # TODO(LLVM-17) Remove this clang-tidy-16 work-around
     if std == "c++23":
         std = "c++2b"
     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
 
 
 _allModules = ["none", "clang", "std"]