Remove mention of slow retrieval attacks

Slow retrieval attacks were removed from the specification in v1.0.7 of
the TUF specification because the specification itself does not provide
mechanisms to protect against this type of attack:

theupdateframework/specification#111

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

content/security.md

@@ -46,10 +46,6 @@ the client is kept unaware of new files.
  with an endless stream of data, causing harm to clients (e.g. a disk partition
 filling up or memory exhaustion).
 
-* **Slow retrieval attacks**. An attacker responds to clients with a very
-slow stream of data that essentially results in the client never completing
-the update process.
-
 * **Extraneous dependencies attacks**. An attacker indicates to clients that,
  in order to install the software they want, they also need to install
  unrelated software. This extra software may be from a trusted source,