ref(insights): remove performance-landing-page-stats-period flag

[DominikB2014]

Just doing a bit of cleanup. This feature has been GA almost 2 years now, no need to keep the flag and it's conditions around.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
9628	1	9627	4

View the top 1 failed test(s) by shortest run time

Performance > Landing > Index renders all transactions view

Stack Traces | 0.632s run time

Error: expect(jest.fn()).toHaveBeenNthCalledWith(n, ...expected)

n: 1
Expected: Anything, ObjectContaining {"query": ObjectContaining {"environment": [], "interval": "1h", "partial": "1", "project": [], "query": "event.type:transaction", "referrer": "api.performance.generic-widget-chart.user-misery-area", "statsPeriod": "14d", "yAxis": ["user_misery()", "tpm()", "failure_rate()"]}}
Received: ".../organizations/org-slug/events-stats/", {"error": [Function error], "query": {"environment": [], "interval": "1h", "partial": "1", "project": [], "query": "event.type:transaction", "referrer": "api.performance.generic-widget-chart.user-misery-area", "statsPeriod": "28d", "yAxis": ["user_misery()", "tpm()", "failure_rate()"]}, "success": [Function success]}

Number of calls: 1
    at Object.<anonymous> (.../performance/landing/index.spec.tsx:204:28)

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