misc: Removed old contribution guidelines regarding branches

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This is old, outdated information. It has largely been replaced by: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>