Remove process_count option

[ashishbista]

Fixes #1148

[eliasjpr]

@elorest I think we should consider removing this ASAP and instead provide an alternative approach for the folks that still needs this.

Example: Alternatively this can be implemented as:

Amber.env = "production"
Amber::Server.start

System.cpu_count.times do |i|
  Process.fork do
    Amber::Server.start
  end
end

sleep

[crimson-knight]

Since multithreading is so nowhere near being considered ready for production, I'm going to close this out and we'll leave the clustering mode the way it is currently.