Remove var/torbrowser_legacy_version from rbm.conf in alpha/nightly
View options
We added var/torbrowser_legacy_version and
var/torbrowser_legacy_platform_version to test the legacy updates in
the alpha branch before the first 14.0 release.
Now that it's being used in the maint-14.0 branch, I think we can
remove the legacy version from the main branch.