rustdoc: clean up some unneeded table lint code

The essential problem is that, with this table:

```text
one |
----|
  a | b | c
  a | b |
  a | b
  a |
```

And this logic:

```rust
let too_many_pipes = divider_count > expected_cells + 1;
```

`expected_cells + 1` winds up as 2, so you get this warning:

```text
error: unused content after last table cell
  --> $DIR/invalid_markdown_table.rs:81:14
   |
LL |     //!   a | b | c
   |              ^^^^^^ this content is discarded

error: unused content after last table cell
  --> $DIR/invalid_markdown_table.rs:83:14
   |
LL |     //!   a | b |
   |              ^^^^ this content is discarded

error: unused content after last table cell
  --> $DIR/invalid_markdown_table.rs:85:14
   |
LL |     //!   a | b
   |              ^^ this content is discarded
```

We really want our warning to give the suggest-escaping flow, like this:

```text
error: table row has too many columns
  --> $DIR/invalid_markdown_table.rs:81:13
   |
LL |     //!   a | b | c
   |             ^ any content after this column divider is discarded
   |
   = help: to escape `|` characters in tables, add a `\` before them like `\|`

error: table row has too many columns
  --> $DIR/invalid_markdown_table.rs:83:13
   |
LL |     //!   a | b |
   |             ^ any content after this column divider is discarded
   |
   = help: to escape `|` characters in tables, add a `\` before them like `\|`

error: unused content after last table cell
  --> $DIR/invalid_markdown_table.rs:85:14
   |
LL |     //!   a | b
   |              ^^ this content is discarded
```

By only scanning the text between the end of the last cell and the row,
instead of doing the entire row, we don't have to re-implement as much
of pulldown-cmark's logic.