Subnet intersection check is preventing legit scenarios

### Description

This has been brought up in discussions:
- #3430
- #4082

And is also one of the issues in #4279.

In a shell:
- this check was added in #1245 though it is no longer clear what was the motivation
- this is preventing macvlan scenarios using the parent subnet for example
- this is at odds with docker behavior

Part of one of the discussions https://github.com/containerd/nerdctl/discussions/3430#discussioncomment-13045708 suggests removing the check entirely.

cc @compdude22 who participated in that discussion.

### Steps to reproduce the issue

Create a network which subnet is overlapping with an existing system subnet.

### Describe the results you received and expected

Expect: work.
Received: creation fail because subnet intersect with system subnet

### What version of nerdctl are you using?

main

### Are you using a variant of nerdctl? (e.g., Rancher Desktop)

None

### Host information

_No response_