Remove links to outdated pep 458

The first removed link is supposed to provide additional reasons
for the need of consistent snapshots. Besides pointing to a removed
anchor in an outdated and unmaintained version of pep 485, the spec
already lists the same reasons that are mentioned in the pep.

The second link points to a different (existing) section in the
same outdated document, but gives implementation details that IMO
don't need to be in the spec. If wished we can update the second
link instead of removing it.

Author: lukpueh

tuf-spec.md

@@ -1304,9 +1304,7 @@ non-volatile storage as FILENAME.EXT.
    high-level idea of the solution is that each snapshot will be contained in a
    so-called consistent snapshot. If a client is reading from one consistent
    snapshot, then the repository is free to write another consistent snapshot
-   without interrupting that client. For more reasons on why we need consistent
-   snapshots, please see
-   https://github.com/theupdateframework/pep-on-pypi-with-tuf#why-do-we-need-consistent-snapshots
+   without interrupting that client.
 
 * **7.1. Writing consistent snapshots**
 
@@ -1358,8 +1356,6 @@ non-volatile storage as FILENAME.EXT.
     released versions of root metadata files should always be provided
     so that outdated clients can update to the latest available root.
 
-    For more details on how this would apply on a community repository, please
-    see https://github.com/theupdateframework/pep-on-pypi-with-tuf#producing-consistent-snapshots
 
 * **7.2. Reading consistent snapshots**