C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/serverenvironment.h

@@ -17,8 +17,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
-#ifndef SERVER_ENVIRONMENT_HEADER
-#define SERVER_ENVIRONMENT_HEADER
+#pragma once
 
 #include "environment.h"
 #include "mapnode.h"
@@ -437,5 +436,3 @@ private:
 	std::unordered_map<u32, float> m_particle_spawners;
 	std::unordered_map<u32, u16> m_particle_spawner_attachments;
 };
-
-#endif