build: Remove unused Less_Output::isEmpty of private class

Introduced in 2014 with commit b6df4c2. It was not used there,
and it is not used now.

Found via https://doc.wikimedia.org/cover/mediawiki-libs-less.php/Output.php.html
as method that lacked test coverage.

Change-Id: I7efb04d4b1af19d6641303f7359f42a2d4c2b9af

Author: Krinkle

lib/Less/Output.php

@@ -25,15 +25,6 @@ public function add( $chunk, $fileInfo = null, $index = 0, $mapLines = null ) {
 		$this->strs[] = $chunk;
 	}
 
-	/**
-	 * Is the output empty?
-	 *
-	 * @return bool
-	 */
-	public function isEmpty() {
-		return count( $this->strs ) === 0;
-	}
-
 	/**
 	 * Converts the output to string
 	 *