misc: Remove mention of ALPHA from the README.
misc: Remove mention of ALPHA from the README.

Change-Id: Ic0faf22e5ed94cf7e7591175a808c4696de29e25
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25462
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5