Fix lost of items in transfer

[cedric-anne]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	-

1. Fix conditions on deletion (see explaination above)
2. Fix typo in table name
3. Fix condition in search for documents to transfer
4. Fix bad usage of key 'dtype' instead of $dtype

Explaination of point 1.

Looks like the base purpose of the code commented with // Clean DB in transfer class was to clean the lines that were not valid (i.e. having a foreign key pointing to a wrong element).

SELECT `glpi_documents_items`.`id`, `glpi_changes`.`id` FROM `glpi_documents_items` LEFT JOIN `glpi_changes` ON (`glpi_documents_items`.`items_id` = `glpi_changes`.`id` AND `glpi_documents_items`.`itemtype` = 'Change') WHERE `glpi_changes`.`id` IS NULL
will return a line with `glpi_changes`.`id` => NULL for all documents that are not linked to a Change.

SELECT `glpi_documents_items`.`id`, `glpi_changes`.`id` FROM `glpi_documents_items` LEFT JOIN `glpi_changes` ON (`glpi_documents_items`.`items_id` = `glpi_changes`.`id`) WHERE `glpi_changes`.`id` IS NULL AND `glpi_documents_items`.`itemtype` = 'Change'
will return a line with `glpi_changes`.`id` => NULL for all documents that are linked to Change itemtype but with an id that does not correspond to a change.