branchmap: remove the droppednodes logic
It was unused. note how it is only extended if the list is empty. So it's always
empty at the end.
We could try to fix that, however this would part of the code is to be removed
in the next changeset as we do not run `branchmap` on truncated repo since
`repoview` in 2.5.
....................
----------------------------------------------------------------------
Ran 20 tests in 0.000s
OK