Drop defunct IS_TRIVIALLY_CONSTRUCTIBLE handling from prevector.h

It's now only referenced from the bench, so leave it there. This allows us to
drop the associated includes as well.

src/bench/prevector.cpp

@@ -2,13 +2,21 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include <compat.h>
 #include <prevector.h>
 #include <serialize.h>
 #include <streams.h>
+#include <type_traits>
 
 #include <bench/bench.h>
 
+// GCC 4.8 is missing some C++11 type_traits,
+// https://www.gnu.org/software/gcc/gcc-5/changes.html
+#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 5
+#define IS_TRIVIALLY_CONSTRUCTIBLE std::has_trivial_default_constructor
+#else
+#define IS_TRIVIALLY_CONSTRUCTIBLE std::is_trivially_default_constructible
+#endif
+
 struct nontrivial_t {
     int x;
     nontrivial_t() :x(-1) {}

src/compat.h

@@ -10,16 +10,6 @@
 #include <config/bitcoin-config.h>
 #endif
 
-#include <type_traits>
-
-// GCC 4.8 is missing some C++11 type_traits,
-// https://www.gnu.org/software/gcc/gcc-5/changes.html
-#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 5
-#define IS_TRIVIALLY_CONSTRUCTIBLE std::has_trivial_default_constructor
-#else
-#define IS_TRIVIALLY_CONSTRUCTIBLE std::is_trivially_default_constructible
-#endif
-
 #ifdef WIN32
 #ifdef _WIN32_WINNT
 #undef _WIN32_WINNT

src/prevector.h

@@ -15,8 +15,6 @@
 #include <iterator>
 #include <type_traits>
 
-#include <compat.h>
-
 #pragma pack(push, 1)
 /** Implements a drop-in replacement for std::vector<T> which stores up to N
  *  elements directly (without heap allocation). The types Size and Diff are