generic ephemeral volume: graduation to GA

The feature gate gets locked to "true", with the goal to remove it in two
releases.

All code now can assume that the feature is enabled. Tests for "feature
disabled" are no longer needed and get removed.

Some code wasn't using the new helper functions yet. That gets changed while
touching those lines.

test/e2e/storage/testsuites/ephemeral.go

@@ -119,6 +119,9 @@ func (p *ephemeralTestSuite) DefineTests(driver storageframework.TestDriver, pat
 			eDriver, _ = driver.(storageframework.EphemeralTestDriver)
 		}
 		if pattern.VolType == storageframework.GenericEphemeralVolume {
+			// The GenericEphemeralVolume feature is GA, but
+			// perhaps this test is run against an older Kubernetes
+			// where the feature might be disabled.
 			enabled, err := GenericEphemeralVolumesEnabled(f.ClientSet, f.Timeouts, f.Namespace.Name)
 			framework.ExpectNoError(err, "check GenericEphemeralVolume feature")
 			if !enabled {