EscapeOutput: remove unused variable $end

This commit removes the unnecessary initialization of the variable
`$end` at the beginning of the sniff code. Every single `case` statement
of the `switch` that follows either re-initialize the variable with a
different value or returns without using the variable.

WordPress/Sniffs/Security/EscapeOutputSniff.php

@@ -185,7 +185,6 @@ public function getGroups() {
 	 */
 	public function process_token( $stackPtr ) {
 		$start = ( $stackPtr + 1 );
-		$end   = $start;
 
 		switch ( $this->tokens[ $stackPtr ]['code'] ) {
 			case \T_STRING: