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

spec/ruby/core/process/exec_spec.rb

@@ -29,16 +29,8 @@ describe "Process.exec" do
     end
   end
 
-  platform_is_not :openbsd do
-    it "raises Errno::EACCES when passed a directory" do
-      -> { Process.exec File.dirname(__FILE__) }.should raise_error(Errno::EACCES)
-    end
-  end
-
-  platform_is :openbsd do
-    it "raises Errno::EISDIR when passed a directory" do
-      -> { Process.exec File.dirname(__FILE__) }.should raise_error(Errno::EISDIR)
-    end
+  it "raises Errno::EACCES when passed a directory" do
+    -> { Process.exec File.dirname(__FILE__) }.should raise_error(Errno::EACCES)
   end
 
   it "runs the specified command, replacing current process" do