ena: Fix customer metrics deallocation statement place

Upstream commit [1] made if_alloc_domain() never fail, then also do the
wrappers if_alloc(), if_alloc_dev(), and if_gethandle().

Upstream commit [2] removed the NULL check conducted by the driver.
This commit also removes err_customer_metrics_alloc goto label.

Commit [2] leaves behind a floating free() statement that
deallocates customer_metrics_array. This commit places the
deallocation statement where it belongs.

[1] commit 4787572d0580 ("ifnet: make if_alloc_domain() never fail")
[2] commit aa3860851b9f ("net: Remove unneeded NULL check for the allocated ifnet")

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.