removing "Cancel" & "Save" buttons for ETH networks that have already been created and can't be edited

[kjozwiak]

Description

When going into brave://wallet --> Settings --> Networks, you'll notice that there's Cancel & Save buttons for each network that's already created by default. However, you can't edit those networks. We should only be displaying Cancel & Save for custom networks that have been added by going through Add Network.

Steps to Reproduce

1. launch 0.69.128 CR: 77.0.3865.75 and open brave://wallet
2. click on Create under New Local Wallet
3. enter your password and click on Remind me later under Secret Backup Phrase
4. click on the My Accounts avatar --> Settings --> Networks

You'll notice Cancel & Save buttons that can't be used for the networks that have already been created.

Actual result:

Expected result:

Shouldn't be displaying Cancel & Save buttons for the networks that have already been created as you cannot edit them.

Reproduces how often:

100% reproducible using the above STR.

Brave version (brave://version info)

Brave	0.69.128 Chromium: 77.0.3865.75 (Official Build) (64-bit)
Revision	201e747d032611c5f2785cae06e894cf85be7f8a-refs/branch-heads/3865@{#776}
OS	macOS Version 10.14.6 (Build 18G95)

Version/Channel Information:

• Can you reproduce this issue with the current release? N/A (technically not released)
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the dev channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? N/A
• Does the issue resolve itself when disabling Brave Rewards? N/A
• Is the issue reproducible on the latest version of Chrome? N/A

Miscellaneous Information:

CCing @brave/legacy_qa

[bbondy]

can we upstream too?

[kjozwiak]

Verification PASSED on macOS 10.14.6 x64 using https://github.com/brave/ethereum-remote-client/releases/tag/1.0.10 via the following build:

Brave	0.69.132 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	macOS Version 10.14.6 (Build 18G95)

• ensured that you can't edit/remove the default networks from Settings -> Networks
• ensured that you can create/edit and delete a custom network

Verification PASSED on Win 10 x64 using https://github.com/brave/ethereum-remote-client/releases/tag/1.0.10 via the following build:

Brave	0.69.132 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Windows 10 OS Version 1809 (Build 17763.737)

• ensured that you can't edit/remove the default networks from Settings -> Networks
• ensured that you can create/edit and delete a custom network

[srirambv]

Verification passed on

Brave	0.69.132 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Linux

• Verified existing pre-loaded networks can't be edited using 1.0.10 via sideload
• Verified new network added can be saved/edited/deleted