Remove now unnecessary step.

Thanks @segfault for catching it!

wiki/src/contribute/release_process/thunderbird.mdwn

@@ -13,10 +13,6 @@ the Tails images build process.
    `+force-all-tests` suffix (most of `features/thunderbird.feature`
    is tagged `@fragile`).
 
-1. Enable the corresponding APT overlay:
-
-        ./bin/add-APT-overlay
-
 1. The first time you do this requires some additional steps (WARNING!
    this will download almost 2 GiB of data):