Updating Brave removes repo file

[sasannnn]

Description

Every time I update Brave, post-install script removes the repo file I've put in /etc/apt/sources.list.d/brave-browser.list

Steps to Reproduce

1. Install brave with repo file named brave-browser.list
2. Brave won't update since brave-browser.list has been removed

Brave version (brave://version info)

Brave | 1.0.0 Chromium: 78.0.3904.97 (Official Build) (64-bit)
Revision | 021b9028c246d820be17a10e5b393ee90f41375e-refs/branch-heads/3904@{#859}
OS | Linux

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
  No
• Does the issue resolve itself when disabling Brave Rewards?
  No
• Is the issue reproducible on the latest version of Chrome?
  No

[bsclifton]

cc: @mbacchi @fmarier

[fmarier]

I was able to reproduce this.

I found that the function which removes the repo gets called from the postrm script.

Given that we disabled the addition of that repo in #1084, we also need to disable its removal.

[btlechowski]

Verification passed on

Brave	1.5.106 Chromium: 80.0.3987.122 (Official Build) beta (64-bit)
Revision	cf72c4c4f7db75bc3da689cd76513962d31c7b52-refs/branch-heads/3987@{#943}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#4291

Before installing Brave

After installing Brave