Remove `__rust_force_expr`.

This was added (with a different name) to improve an error message. It
is no longer needed -- removing it changes the error message, but overall
I think the new message is no worse:
- the mention of `#` in the first line is a little worse,
- but the extra context makes it very clear what the problem is, perhaps
  even clearer than the old message,
- and the removal of the note about the `expr` fragment (an internal
  detail of `__rust_force_expr`) is an improvement.

Overall I think the error is quite clear and still far better than the
old message that prompted #61933, which didn't even mention patterns.

The motivation for this is #124141, which will cause pasted
metavariables to be tokenized and reparsed instead of the AST node being
cached. This change in behaviour occasionally has a non-zero perf cost,
and `__rust_force_expr` causes the tokenize/reparse step to occur twice.
Removing `__rust_force_expr` greatly reduces the extra overhead for the
`deep-vector` benchmark.

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