doc: remove stray text in Git data model

I meant to delete this sentence fragment when rewriting this paragraph,
but accidentally left it in. It's repetitive (since it was meant to be
deleted) and it's causing some formatting issues with the note.

Signed-off-by: Julia Evans <julia@jvns.ca>

Author: jvns

Documentation/gitdatamodel.adoc

@@ -235,8 +235,6 @@ there will no longer be a branch that points at the old commit.
 The old commit is recorded in the current branch's <<reflogs,reflog>>,
 so it is still "reachable", but when the reflog entry expires it may
 become unreachable and get deleted.
-
-the old commit will usually not be reachable, so it may be deleted eventually.
 Reachable objects will never be deleted.
 
 [[index]]