chore(rls): Remove passing global username

[john-bodley]

SUMMARY

I'm not well versed in the row level security logic, but it seems there's functionality of the form,

if hasattr(g, "user"):
    user = g.user
elif username:
    user = self.find_user(username=username)
else: 
    return []

to obtain the user associated with the RLS check. The issue is the optional username—which is used to set the user if no current user exists—was only specified once via get_username() which makes the the above logic behave like,

user = self.find_user(username=g.user.username if g.user else None)

which can be then refactored as,

user = g.user if hasattr(g, "user") else None

which makes the elif clause unnecessarily as it's captured in the if clause. Furthermore that g.user and self.find_user(...) can return None and so the else should also replaced with the short circuit logic in essence returning if and only if there is isn't a user defined.

Said logic was added in #19999 but was likely made obsolete by #19914.

This PR removes the optional username argument throughout the RLS code path.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

CI.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #20344 (73ec46b) into master (ec331e6) will increase coverage by 0.06%.
The diff coverage is 61.41%.

❗ Current head 73ec46b differs from pull request most recent head da4fab6. Consider uploading reports for the commit da4fab6 to get more accurate results

@@            Coverage Diff             @@
##           master   #20344      +/-   ##
==========================================
+ Coverage   66.64%   66.70%   +0.06%     
==========================================
  Files        1738     1754      +16     
  Lines       65060    65593     +533     
  Branches     6885     6938      +53     
==========================================
+ Hits        43360    43756     +396     
- Misses      19953    20079     +126     
- Partials     1747     1758      +11     

Flag	Coverage Δ
hive	53.78% <ø> (+0.06%)	⬆️
mysql	?
postgres	82.43% <ø> (+0.07%)	⬆️
presto	?
python	82.70% <ø> (-0.09%)	⬇️
sqlite	82.22% <ø> (+0.12%)	⬆️
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...art-controls/src/operators/contributionOperator.ts	100.00% <ø> (ø)
...ui-chart-controls/src/operators/flattenOperator.ts	100.00% <ø> (ø)
...t-ui-chart-controls/src/operators/pivotOperator.ts	100.00% <ø> (ø)
...ui-chart-controls/src/operators/prophetOperator.ts	100.00% <ø> (ø)
...i-chart-controls/src/operators/resampleOperator.ts	100.00% <ø> (ø)
...controls/src/operators/timeComparePivotOperator.ts	100.00% <ø> (ø)
...ui-core/src/chart/components/FallbackComponent.tsx	100.00% <ø> (ø)
...superset-ui-core/src/query/types/PostProcessing.ts	100.00% <ø> (ø)
.../superset-ui-core/src/query/types/QueryFormData.ts	100.00% <ø> (ø)
...set-ui-core/src/ui-overrides/UiOverrideRegistry.ts	100.00% <ø> (ø)
... and 281 more

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[john-bodley]

ping @betodealmeida

[zhaoyongjie]

LGTM