Experimental 3box feature should be disabled

[ryanml]

At this time you should not be able to toggle on this feature:

Test Plan:

1. Go to Settings > Advanced
2. At the bottom of the options under Auto-Logout Timer (minutes), there should be no option to enable 3box

Current behavior:

[kjozwiak]

Verification PASSED on macOS 10.15.1 x64 using https://github.com/brave/ethereum-remote-client/releases/tag/0.1.32 with the following:

Brave	1.1.22 Chromium: 79.0.3945.79 (Official Build) (64-bit)
Revision	29f75ce3f42b007bd80361b0dfcfee3a13ff90b8-refs/branch-heads/3945@{#916}
OS	macOS Version 10.15.1 (Build 19B88)

---

Verification passed on

Brave	1.2.41 Chromium: 79.0.3945.88 (Official Build) (64-bit)
Revision	c2a58a36b9411c80829b4b154bfcab97e581f1f3-refs/branch-heads/3945@{#954}
OS	Linux
Component	0.1.34

---

Verification passed on

Brave	1.2.41 Chromium: 79.0.3945.88 (Official Build) (64-bit)
Revision	c2a58a36b9411c80829b4b154bfcab97e581f1f3-refs/branch-heads/3945@{#954}
OS	Windows 10 OS Version 1909 (Build 18363.535)
Component	0.1.34