[FIX] pos: remove unused key use in move lines grouping
[FIX] pos: remove unused key use in move lines grouping

The key 'tax_code_id' does not exist anymore, therefore the condition
line.get('tax_code_id') == values.get('tax_code_id')
was equivalent to
None == None
which is of course always True.

To make this code more readable and avoid misunderstandings, a clearer
loop implementation avoids checking for this key.

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