[REM] *: remove all demo.action buttons from the user documentation

- cherry-pick of e10581ef6e (see PR #572 )
- more fields deleted so that there was none left in 12.0

--> this makes the build of 12.0 much faster

*: accounting, inventory, purchase

As these buttons points to demo.odoo.com, it is outdated very quickly
and leads to weird results were a button on the 10.0 documentation
points to demo.odoo.com that runs 13.0 or no longer exists.

accounting/payables/pay/check.rst

@@ -71,14 +71,6 @@ and validate the payment dialog.
 .. image:: ./media/check02.png
   :align: center
 
-Explanation of the fields of the payment screen:
-
-.. demo:fields:: account.action_account_payments
-
-.. demo:action:: account.action_account_payments
-	
-	Try paying a supplier bill with a check
-
 .. _PrintChecks:
 
 Print checks