Revert "Adds note about custom server requirements" (#39956)
Revert "Adds note about custom server requirements" (#39956)

Reverts vercel/next.js#39931

Let's instead add this to the custom server documentation, in a section on "Using Middleware". Since such a low percentage of folks eject out of the default server, this shouldn't be so high up on the Middleware docs page. 
JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js