Don't call `Warning.warn` unless the category is enabled

The warning category should be enabled if we want to call
`Warning.warn`.

This commit speeds up the following benchmark:

```ruby
eval "def test; " +
  1000.times.map { "'  '.chomp!" }.join(";") + "; end"

def run_benchmark count
  i = 0
  while i < count
    start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
    yield
    ms = Process.clock_gettime(Process::CLOCK_MONOTONIC) - start
    puts "itr ##{i}: #{(ms * 1000).to_i}ms"
    i += 1
  end
end

run_benchmark(25) do
  250.times do
    test
  end
end
```

On `master` this runs at about 92ms per iteration. With this patch, it
is 7ms per iteration.

[Bug #20573]