Revert "go-url-shortener: set GOMAXPROCS"

This reverts commit d266614d0bfd989dc5dd93c4a950f90a96ba4870.
Revert "init.go: rely on GOMAXPROCS being set from outside (via env variable)"

This reverts commit 7e2f11ce77a198940034108f432722ed7fec5d59.