remove unneeded ERR_PC_SET

This ERR_PC_SET seems to make wrong backtrace.

```ruby
def a
  begin
    raise ""
  ensure
  end
end
a
```
```
trace:
	[2] test.rb:5:in Object.a
	[0] test.rb:7
test.rb:3: RuntimeError
```
```ruby
begin
  raise ""
ensure
end
```

```
trace:
	[1] test.rb:4
test.rb:4: RuntimeError
```