Remove "mode: preserve" option from documentation (#71486)

Remove "mode: preserve" option from documentation in doc fragments "FILE_COMMON_ARGUMENTS", as it was incorrectly included in the documentation for the `lineinfile`, `unarchive` and other file-related modules. 
The `copy` and `templates` modules documentation remains untouched and still contain "mode: preserve", as intended.

lib/ansible/plugins/doc_fragments/files.py

@@ -25,8 +25,6 @@ options:
       number which will have unexpected results.
     - As of Ansible 1.8, the mode may be specified as a symbolic mode (for example, C(u+rwx) or
       C(u=rw,g=r,o=r)).
-    - As of Ansible 2.6, the mode may also be the special string C(preserve).
-    - When set to C(preserve) the file will be given the same permissions as the source file.
     type: raw
   owner:
     description: