[libc++] Remove unused include in <compare>.

[mumbleskates]

Summary

<type_traits> was included in the first iteration of <compare> when it was created as a monolithic header, then never removed. Removing it now is a beneficial no-op since it is not guaranteed by the standard and is already included by all of its subheaders.

[• Quuxplusone]

Sure, LGTM. For pre-C++20 headers I believe we still don't want to remove #includes this way, because someone in the wild might be relying on the #include. But for C++20 <compare> (and C++20 <concepts> we're going ahead and doing this, AFAIK.

libcxx/CREDITS.TXT

160–163

If you're trying to alphabetize the contributor list, this is laughably insufficient. ;) I recommend reverting this diff, but it doesn't really matter AFAIC.

[mumbleskates]

libcxx/CREDITS.TXT

160–163

oh no, hahaha... i'll leave this as-is to grease the rails and fix the other misalphabetization in another diff later.

[mumbleskates]

Remove CREDITS.TXT changes (see https://reviews.llvm.org/D108263 )

libcxx/CREDITS.TXT

 N: Nico Rieck
 E: [e-mail]
 D: Windows fixes
 
 N: Jon Roelofs
 E: [e-mail]
 D: Remote testing, Newlib port, baremetal/single-threaded support.
 
+N: Kent Ross
+E: [e-mail]
+D: C++20 language feature support
+
 N: Jonathan Sauer
 D: Minor patches, mostly related to constexpr
 
 N: Craig Silverstein
 E: [e-mail]
 D: Implemented Cityhash as the string hash function on 64-bit machines
 
 N: Richard Smith

 N: Zhang Xiongpang
 E: [e-mail]
 D: Minor patches and bug fixes.
 
 N: Xing Xue
 E: [e-mail]
 D: AIX port
 
-N: Zhihao Yuan
-E: [e-mail]
-D: Standard compatibility fixes.
-
 N: Jeffrey Yasskin
 E: [e-mail]
 E: [e-mail]
 D: Linux fixes.
+
+N: Zhihao Yuan
+E: [e-mail]
+D: Standard compatibility fixes.

libcxx/include/compare

public:
     friend constexpr strong_ordering operator<=>(unspecified, strong_ordering v) noexcept;
   };
 }
 */
 
 #include <__compare/common_comparison_category.h>
 #include <__compare/ordering.h>
 #include <__config>
-#include <type_traits>
 
 #ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR)