chore: default_roles hook doesn't exist anymore

[blaggacao]

Context

The default_roles hook as described in https://frappeframework.com/docs/user/en/guides/portal-development/portal-roles doesn't exist anymore in Frappe.

Proposed Solution

• Remove it's noop declaration
• In order to not confuse people who read the code
• Update the docs (which I seem not to be able to in this PR)

[codecov]

Codecov Report

Attention: 19 lines in your changes are missing coverage. Please review.

Comparison is base (33b631e) 60.03% compared to head (c9a4ae6) 60.10%.
Report is 14 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #39032      +/-   ##
===========================================
+ Coverage    60.03%   60.10%   +0.06%     
===========================================
  Files          761      761              
  Lines        70122    70188      +66     
===========================================
+ Hits         42099    42184      +85     
+ Misses       28023    28004      -19     

Files	Coverage Δ
...xt/accounts/doctype/sales_invoice/sales_invoice.py	74.49% <100.00%> (+0.01%)	⬆️
...next/accounts/doctype/subscription/subscription.py	77.97% <100.00%> (+0.25%)	⬆️
erpnext/controllers/sales_and_purchase_return.py	91.18% <100.00%> (+0.57%)	⬆️
erpnext/hooks.py	100.00% <ø> (ø)
erpnext/manufacturing/doctype/bom/bom.py	73.32% <100.00%> (+0.75%)	⬆️
...pnext/stock/doctype/delivery_note/delivery_note.py	66.39% <100.00%> (+0.05%)	⬆️
erpnext/stock/doctype/stock_entry/stock_entry.py	79.22% <100.00%> (-0.02%)	⬇️
erpnext/controllers/accounts_controller.py	85.27% <85.71%> (+<0.01%)	⬆️
...e/manufacturing_settings/manufacturing_settings.py	36.36% <0.00%> (-1.14%)	⬇️
erpnext/stock/doctype/warehouse/warehouse.py	67.78% <80.00%> (+0.19%)	⬆️
... and 5 more

... and 1 file with indirect coverage changes