gdb: Use initializers in lambda captures unconditionally

Initializer in lambda captures were introduced in C++14, and
conditionally used in gdb/cp-support.c and gdb/dwarf2/cooked-index.c.

Since C++17 is now required by GDB, use this feature unconditionally.

Change-Id: I87a3d567941e5c71217538fa75c952e4d421fa1d

gdb/cp-support.c

@@ -1658,13 +1658,7 @@ gdb_demangle (const char *name, int options)
 	     we might be in a background thread.  Instead, arrange for
 	     the reporting to happen on the main thread.  */
 	  std::string copy = name;
-	  run_on_main_thread ([
-#if __cplusplus >= 201402L
-			       =, copy = std::move (copy)
-#else
-			       =
-#endif
-			       ] ()
+	  run_on_main_thread ([=, copy = std::move (copy)] ()
 	    {
 	      report_failed_demangle (copy.c_str (), core_dump_allowed,
 				      crash_signal);

gdb/dwarf2/cooked-index.c

@@ -466,12 +466,7 @@ cooked_index::start_writing_index (dwarf2_per_bfd *per_bfd)
      started, because it may call 'wait'.  */
   m_write_future
     = gdb::thread_pool::g_thread_pool->post_task ([this, per_bfd,
-#if __cplusplus >= 201402L
-						   ctx = std::move (ctx)
-#else
-						   ctx
-#endif
-						   ] ()
+						   ctx = std::move (ctx)] ()
 	{
 	  maybe_write_index (per_bfd, ctx);
 	});