Remove permissions-lifetime feature flag

[stephendonner]

Description

Remove permissions-lifetime feature flag

Steps to Reproduce

1. new profile
2. load brave://flags
3. type lifetime

Actual result:

Even though it's on by default (from #16646), there's still a feature flag for the permissions-lifetime feature, at brave://flags/#brave-permission-lifetime

Expected result:

There shouldn't be a feature flag for permissions-lifetime available at brave://flags/#brave-permission-lifetime

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.28.74 Chromium: 92.0.4515.93 (Official Build) nightly (x86_64)
Revision	6eb43ff7850a1d710c3f827a0555737c74edab5c-refs/branch-heads/4515@{#1378}
OS	macOS Version 11.4 (Build 20F71)

/cc @goodov

[stephendonner]

Verified FIXED using

Brave	1.29.8 Chromium: 92.0.4515.101 (Official Build) nightly (x86_64)
Revision	edb73f4fb624e2ea0cb6f5cc39c9e317ecd3535b-refs/branch-heads/4515@{#1536}
OS	macOS Version 11.4 (Build 20F71)

Steps:

1. new profile
2. launch Brave
3. load brave://flags
4. search for lifetime
5. load https://permission.site/ and click on Location
6. confirmed the permission was granted to https://permission.site/ for location via site info

example	example