Update disabledContent.js to the modified BEM style

[luixxiul]

Test plan

#10337 (comment)

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Describe the issue you encountered: Update disabledContent.js to the modified BEM style (ref: https://github.com/brave/browser-laptop/blob/master/docs/style.md#defining-our-blocks-elements-and-modifiers).

• Platform (Win7, 8, 10? macOS? Linux distro?): All

• Brave Version (revision SHA): master branch

• Actual result: the left margin between the gray area and the left main sidebar changes slightly. It will be addressed at the same time.

  

• Expected result: it should not change

• Extra QA steps:

  1. Open about:preferences#payments
  2. Disable Payments
  3. Make sure the left and right rows are displayed properly
  4. Enable Payments
  5. Make sure the left margin of the gray block does not change

• Screenshot if needed:
  it should look like this:
  

• Any related issues: Fix BEM naming on enabledContent.js #10146