Remove unused

MultireasonException has been replaced with CompositeException.

lib/MultiReasonException.php

@@ -1,29 +0,0 @@
-<?php
-
-namespace Amp;
-
-class MultiReasonException extends \Exception
-{
-    /** @var \Throwable[] */
-    private array $reasons;
-
-    /**
-     * @param \Throwable[] $reasons Array of exceptions rejecting the promise.
-     * @param string|null  $message
-     */
-    public function __construct(array $reasons, string $message = null)
-    {
-        parent::__construct($message ?: "Multiple errors encountered; use "
-            . self::class . "::getReasons() to retrieve the array of exceptions thrown");
-
-        $this->reasons = $reasons;
-    }
-
-    /**
-     * @return \Throwable[]
-     */
-    public function getReasons(): array
-    {
-        return $this->reasons;
-    }
-}