* Fix shadowed variable bug

This code caused the singleton `trustedRoot` to be returned as nil on subsequent calls. The singleton was shadowed when the variable was redeclared in the `if` block.

Signed-off-by: Cody Soyland <codysoyland@github.com>

* Remove unused singleton

`singletonRootError` was never returned without being overwritten, so it was essentially unused. I think it's wise to always retry the TUF call on future invocations in case of network errors.

Signed-off-by: Cody Soyland <codysoyland@github.com>

---------

Signed-off-by: Cody Soyland <codysoyland@github.com>