allow purge expired STS while loading credentials (#19905)

the reason for this is to avoid STS mappings to be
purged without a successful load of other policies,
and all the credentials only loaded successfully
are properly handled.

This also avoids unnecessary cache store which was
implemented earlier for optimization.

cmd/sts-handlers_test.go

@@ -50,7 +50,6 @@ func runAllIAMSTSTests(suite *TestSuiteIAM, c *check) {
 }
 
 func TestIAMInternalIDPSTSServerSuite(t *testing.T) {
-	t.Skip("FIXME: Skipping internal IDP tests. Flaky test, needs to be fixed.")
 	baseTestCases := []TestSuiteCommon{
 		// Init and run test on ErasureSD backend with signature v4.
 		{serverType: "ErasureSD", signer: signerV4},