Remove obsolete Brillo variable

Unlike the original change on master, this does not remove
product_variables.brillo, as I'm not backporting all of the other
removal CLs.

Bug: 76168832
Test: none
Change-Id: I6a5ce57b317f0cdae1abef15def01e6a31e18d3e
Merged-In: I6a5ce57b317f0cdae1abef15def01e6a31e18d3e
(cherry picked from commit a052599b)