stdlib: Remove stdlib README.md
stdlib: Remove stdlib README.md

This README.md is outdated and incompleted. User's wishing to learn
about the gem5 stdlib should reference the gem5 website:
https://www.gem5.org/documentation/gem5-stdlib/overview

Issue-on: https://gem5.atlassian.net/browse/GEM5-1019
Change-Id: Ib66bd748d517708833ac591515601f206ce4728a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55323
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56244

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