Restore sanity to line continuations

Line continuations (i.e. escaped new lines) now make sense again. With
the smart pipe support (pipes continue on to next line) recently added,
this hack to have continuations ignore comments makes no sense.

This is valid code:

```fish
echo hello |
# comment here
tr -d 'l'
```

this isn't:
```fish
echo hello | \
# comment here
tr -d 'l'
```

Reverts @snnw's 318daaffb2
Closes #2928. Closes #2929.

src/tokenizer.h

@@ -104,8 +104,6 @@ class tokenizer_t {
     bool show_comments{false};
     /// Whether all blank lines are returned.
     bool show_blank_lines{false};
-    /// Whether to continue the previous line after the comment.
-    bool continue_line_after_comment{false};
 
     tok_t call_error(tokenizer_error *error_type, const wchar_t *token_start,
                      const wchar_t *error_loc);