gracefully handle deleteFromSelf when share is already gone

- handling race conditions

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

lib/private/Share20/DefaultShareProvider.php

@@ -470,7 +470,8 @@ class DefaultShareProvider implements IShareProvider {
 			}
 
 			if (!$group->inGroup($user)) {
-				throw new ProviderException('Recipient not in receiving group');
+				// nothing left to do
+				return;
 			}
 
 			// Try to fetch user specific share

tests/lib/Share20/DefaultShareProviderTest.php

@@ -1558,9 +1558,6 @@ class DefaultShareProviderTest extends \Test\TestCase {
 
 
 	public function testDeleteFromSelfGroupUserNotInGroup() {
-		$this->expectException(\OC\Share20\Exception\ProviderException::class);
-		$this->expectExceptionMessage('Recipient not in receiving group');
-
 		$qb = $this->dbConn->getQueryBuilder();
 		$stmt = $qb->insert('share')
 			->values([