Delete comment char when joining commented lines

Let's say we have the following Ruby file.

```rb
# First line of Ruby comments,
# and second line of Ruby comments
```

Now our cursor is on the first line, and we press `J`.

Before:

```rb
# First line of Ruby comments, # and second line of Ruby comments
```

After:

```rb
# First line of Ruby comments, and second line of Ruby comments
```