Update account-hierarchy-permissions.md

[chirschklau]

Removed SignupCustomer limitation as it no longer applies.

[opbld32]

Docs Build status updates of commit 761e277:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
Advertising/bingads-13/guides/account-hierarchy-permissions.md	:white_check_mark:Succeeded	View (=bingads-13)

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