Remove Acacia testnet for identity creation and import

[aurelticot]

💭 What's changed?

• Removed the feature flag to disable the Mainnet as it was more of a kill switch for during the Mainnet launch
• Remove the Acacia testnet from the list of supported Network

🧪 How to test these changes?

• Create a new identity and check the testnet is not available anymore
• Import a new identity and check the testnet is not available anymore

😨 Anything to be aware of?

• Further work on removing the testnet will be conducted in Update Verida SDK to v4 + handle new named networks #1396
• It's only disabling the creation/import. Existing testnet identity in the wallet should continue to work until the Network effectively goes down, from there, the work on Adapt the "Not found identity" modal to handle the Acacia shutdown #1383 would allow removing them

[andy-verida]

LGTM