spec/ruby/core/process/exec_spec.rb: remove a guard for openbsd
spec/ruby/core/process/exec_spec.rb: remove a guard for openbsd

openbsd current seems to behave the same as other OSs.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20200305T063005Z.fail.html.gz

eregon/rubyspec,eregon/rubyspec,eregon/rubyspec,nobu/rubyspec,nobu/rubyspec,nobu/rubyspec,ruby/spec,ruby/spec,ruby/spec