Remove Export option for Zcash accounts

[srirambv]

Description

Remove Export option for Zcash accounts

Steps to Reproduce

1. Enable flag and create a Zcash account
2. Open Accounts tab and select Zcash
3. Click on account menu, shows Export which should not be shown

Actual result:

Expected result:

No export option similar to BTC

Reproduces how often:

Easy

Brave version (brave://version info)

Brave	1.64.51 Chromium: 121.0.6167.139 (Official Build) nightly (64-bit)
Revision	f5082bd
OS	Linux

Version/Channel Information:

• Can you reproduce this issue with the current release? NA
• Can you reproduce this issue with the beta channel? NA
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

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

Miscellaneous Information:

cc: @cypt4

[kjozwiak]

The above requires 1.63.165 or higher for 1.63.x verification 👍

[srirambv]

Verification passed on

Brave	1.63.165 Chromium: 122.0.6261.94 (Official Build) (64-bit)
Revision	fed0fcc915f6d9e1db871202f0304c8d582a73de
OS	Windows 11 Version 23H2 (Build 22631.3155)

• Verified steps from brave/brave-core#22120
• Verified Export account option is not shown for BTC/ZEC accounts

35849.mp4