Revert "Add WebKit to e2e test matrix (#37298)" (#37315)

Reverts: #37298
Ref:
https://github.com/go-gitea/gitea/actions/runs/24661464168/job/72108324223?pr=37312

WebKit on Linux has a long history of flakiness in Playwright CI runs,
and the exact "WebKit encountered an internal error" class of failures
has been reported repeatedly and closed without a real fix (typically
stale/no-repro, or worked around by retries):

- https://github.com/microsoft/playwright/issues/34450
- https://github.com/microsoft/playwright/issues/35773
- https://github.com/microsoft/playwright/issues/35870
- https://github.com/microsoft/playwright/issues/35293
- https://github.com/microsoft/playwright/issues/38838

Keeping chromium and firefox in the e2e matrix.

---
This PR was written with the help of Claude Opus 4.7

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

Makefile

@@ -518,7 +518,8 @@ test-mssql-migration: migrations.mssql.test migrations.individual.mssql.test
 
 .PHONY: playwright
 playwright: deps-frontend
-	@pnpm exec playwright install --with-deps chromium firefox webkit $(PLAYWRIGHT_FLAGS)
+	@# on GitHub Actions VMs, playwright's system deps are pre-installed
+	@pnpm exec playwright install $(if $(GITHUB_ACTIONS),,--with-deps) chromium firefox $(PLAYWRIGHT_FLAGS)
 
 .PHONY: test-e2e
 test-e2e: playwright $(EXECUTABLE_E2E)