Xcode: Drop hard-coded default warning flags like -Wmost

The `-Wmost` flag was added by commit 97a51d2172 (ENH: add more xcode
stuff, 2005-01-27, v2.4.0~2396), along with a couple others, as part of
very early work on the Xcode generator.  I suspect that the flags were
part of Xcode's project creation wizard.  However, they are not
appropriate for CMake-based projects because we already have our own
mechanisms for specifying flags desired by the project or user.
Simply drop the flags.

Fixes: #13516

Help/release/dev/xcode-default-warnings.rst

@@ -0,0 +1,5 @@
+xcode-default-warnings
+----------------------
+
+* The :generator:`Xcode` generator no longer hard-codes ``-Wmost``,
+  ``-Wno-four-char-constants``, and ``-Wno-unknown-pragmas`` warning flags.

Source/cmGlobalXCodeGenerator.cxx

@@ -2357,9 +2357,6 @@ void cmGlobalXCodeGenerator::CreateBuildSettings(cmGeneratorTarget* gtgt,
   buildSettings->AddAttribute("SECTORDER_FLAGS", this->CreateString(""));
   buildSettings->AddAttribute("USE_HEADERMAP", this->CreateString("NO"));
   cmXCodeObject* group = this->CreateObject(cmXCodeObject::OBJECT_LIST);
-  group->AddObject(this->CreateString("-Wmost"));
-  group->AddObject(this->CreateString("-Wno-four-char-constants"));
-  group->AddObject(this->CreateString("-Wno-unknown-pragmas"));
   group->AddObject(this->CreateString("$(inherited)"));
   buildSettings->AddAttribute("WARNING_CFLAGS", group);