misc: Removed old contribution guidelines regarding branches

This is old, outdated information. It has largely been replaced by:
https://gem5-review.googlesource.com/c/public/gem5/+/24263

Change-Id: I2f232cd1b3133f94434f4f12b1206dabaf339899
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24803
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index e7c743e..9e34f60 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -78,19 +78,6 @@
 community. Additionally, the maintainer for that part of the code must sign off
 on it.
 
-Contributing long-lived feature branches
-----------------------------------------
-Oftentimes users or institutions add features that are necessarily complex,
-and require many changes on long-lived feature branches. In this case,
-maintaining a perfect history where all changes work individually is infeasible.
-When contributing long-lived feature branches back to gem5's public repository
-users may merge entire long-lived branches into a single changeset and contribute
-their code back as long as 1) the changes have been reviewed by the maintainer
-2) the maintainer agrees to allow such a change, and 3) the changes are passing
-the public tests. Changes that affect common code (outside of a specific
-maintainer's purview) will still need to follow the standard gem5 protocol.
-
-
 Cloning the gem5 repo to contribute
 ===================================
 
@@ -111,10 +98,6 @@
  * public/m5threads: The code for a pthreads implementation that works with
    gem5's syscall emulation mode.
 
-Other gem5 branches
--------------------
-
-None right now.
 
 Making changes to gem5
 ======================