Remove mention of required memory to build
Remove mention of required memory to build

Because it, obviously, changes all the time and 600MiB is way
out-of-date now.

graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust