build: No need to check for leveldb atomics

They're guaranteed with c++11

configure.ac

@@ -556,18 +556,6 @@ if test x$use_reduce_exports = xyes; then
   [AC_MSG_ERROR([Cannot set default symbol visibility. Use --disable-reduce-exports.])])
 fi
 
-dnl This can go away when we require c++11
-TEMP_CXXFLAGS="$CXXFLAGS"
-CXXFLAGS="$CXXFLAGS -std=c++0x"
-AC_MSG_CHECKING(for c++11 atomics)
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
-  #include <atomic>
-  ]],[[]])],
-  [ AC_MSG_RESULT(yes); LEVELDB_ATOMIC_CPPFLAGS="-DLEVELDB_ATOMIC_PRESENT"; LEVELDB_ATOMIC_CXXFLAGS="-std=c++0x"],
-  [ AC_MSG_RESULT(no)]
-)
-CXXFLAGS="$TEMP_CXXFLAGS"
-
 LEVELDB_CPPFLAGS=
 LIBLEVELDB=
 LIBMEMENV=
@@ -1072,8 +1060,6 @@ AC_SUBST(TESTDEFS)
 AC_SUBST(LEVELDB_TARGET_FLAGS)
 AC_SUBST(MINIUPNPC_CPPFLAGS)
 AC_SUBST(MINIUPNPC_LIBS)
-AC_SUBST(LEVELDB_ATOMIC_CPPFLAGS)
-AC_SUBST(LEVELDB_ATOMIC_CXXFLAGS)
 AC_CONFIG_FILES([Makefile src/Makefile share/setup.nsi share/qt/Info.plist src/test/buildenv.py])
 AC_CONFIG_FILES([qa/pull-tester/run-bitcoind-for-test.sh],[chmod +x qa/pull-tester/run-bitcoind-for-test.sh])
 AC_CONFIG_FILES([qa/pull-tester/tests_config.py],[chmod +x qa/pull-tester/tests_config.py])