ConsoleCommandSender: Remove useless function isPlayer()

This function is not declared in any useful places (like the CommandSender interface) and it is not present in Player (!!!). Additionally, an is-player check is better done with an instanceof so that type safety is enforced and IDEs can give auto-complete.

This is a BC break, but this is such a pointless function that it's probably not even worth mentioning.

src/pocketmine/command/ConsoleCommandSender.php

@@ -92,13 +92,6 @@ class ConsoleCommandSender implements CommandSender{
 		return $this->perm->getEffectivePermissions();
 	}
 
-	/**
-	 * @return bool
-	 */
-	public function isPlayer() : bool{
-		return false;
-	}
-
 	/**
 	 * @return Server
 	 */