Fix EZP-16372: deleting of content lasts very long

The prototypes of `removeObject()` and `removeObjectById()` differ between `eZSearch` and `eZFind`, i.e:

`static function removeObject( $contentObject, $commit = true )`
versus
`function removeObject( $contentObject, $commit = null )`

When `eZContentObject` calls `eZSearch::removeObjectById( $delID )`, `removeObjectById()` is called by `eZSearch` with its default value, `$commit = true`.

So the check :
`if ( !isset( $commit ) && ( $this->FindINI->variable( 'IndexOptions', 'DisableDeleteCommits' ) === 'true' ) )`

is always false, because `$commit` is always defined.

So unify the prototypes between them, setting $commit default value to null.