WordPress 4.4 introduced core support for emojis. This automatically converts emoticons in your content from their text representation to an icon.
Fortunately it can be removed quite easily by running the following php code.
If you’re not sure where to place this code I highly recommend you read How to create a WordPress plugin for your custom functions.
remove_action( 'wp_head', 'print_emoji_detection_script', 7 ); remove_action( 'wp_print_styles', 'print_emoji_styles' );
For more information on emojis in WordPress see: https://codex.wordpress.org/Using_Smilies