etcdmain: stop setting GOMAXPROCS explicitly
etcdmain: stop setting GOMAXPROCS explicitly

We always want to use GOMAXPROCS() as the way go parses it. When in go1.4, we
want to expose GOMAXPROCS value, so we set GOMAXPROCS explicitly as the
way go 1.4 does and print it out.

But it becomes a problem when go 1.5 changes the way to set GOMAXPROCS.

Fix the problem by stop setting GOMAXPROCS and get its value directly.

Due to this change, it sets default GOMAXPROCS to the
number of CPUs available when compiling in go 1.5, which matches how go 1.5 works:
https://docs.google.com/document/d/1At2Ls5_fhJQ59kDK2DFVhFu3g5mATSXqqV5QrxinasI/edit

This is a behavior change in etcd 2.2.

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