[native] Remove network.json fallbacks

Summary:
Addresses ENG-2742. ENG-2743

Autodetected IP has been staying with us for some time without issues, time to finally remove network.json.

Steps made:

• Removed native/facts/network.json references as described in ENG-2742. Deleting it from .gitignore will force people still having this file to remove it
• Reworked the warning as described in ENG-2743.

Test Plan:

• Ran yarn dev and opened mobile app. Ensured that it keeps working as expected.
• Manually replaced ip.v4.sync() with undefined and ensured that:
  • The warning is displayed once in Metro
  • The warning is displayed once in-app
• When the COMM_NAT_DEV_HOSTNAME env variable is set, the above warnings disappear

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7056