Remove unnecessary assert calls

[GromNaN]

Q	A
Type	improvement
BC Break	no
Fixed issues	-

Summary

Fix errors reported by PHPStan.

 ------ ---------------------------------------------------------------------------------------- 
  Line   lib/Doctrine/ODM/MongoDB/Aggregation/Aggregation.php                                    
 ------ ---------------------------------------------------------------------------------------- 
  60     Ignored error pattern #^Call to function assert\(\) with true will                      
         always evaluate to true\.$# (function.alreadyNarrowedType) in path                      
         /Users/jerome/Develop/mongodb-odm/lib/Doctrine/ODM/MongoDB/Aggregation/Aggregation.php  
         is expected to occur 1 time, but occurred 2 times.                                      
  61     Call to function assert() with true will always evaluate to true.                       
         🪪 function.alreadyNarrowedType                                                         
  61     Instanceof between MongoDB\Driver\CursorInterface and Iterator will                     
         always evaluate to true.                                                                
         🪪 instanceof.alwaysTrue                                                                
 ------ ---------------------------------------------------------------------------------------- 

 ------ --------------------------------------------------------------------------------------------- 
  Line   lib/Doctrine/ODM/MongoDB/Persisters/DocumentPersister.php                                    
 ------ --------------------------------------------------------------------------------------------- 
  347    Ignored error pattern #^Call to function assert\(\) with true will                           
         always evaluate to true\.$# (function.alreadyNarrowedType) in path                           
         /Users/jerome/Develop/mongodb-odm/lib/Doctrine/ODM/MongoDB/Persisters/DocumentPersister.php  
         is expected to occur 1 time, but occurred 2 times.                                           
  547    Call to function assert() with true will always evaluate to true.                            
         🪪 function.alreadyNarrowedType                                                              
  547    Instanceof between MongoDB\Driver\CursorInterface and Iterator will                          
         always evaluate to true.                                                                     
         🪪 instanceof.alwaysTrue                                                                     
  547    Result of && is always true.                                                                 
         🪪 booleanAnd.alwaysTrue                                                                     
 ------ --------------------------------------------------------------------------------------------- 

[GromNaN]

All the iterators (CachingIterator, HydratingIterator and UnrewindableIterator) take a Transversable. They don't need that we assert the SPL Iterable type.

[GromNaN]

This is a duplicate assert.