Rollup merge of #71634 - eddyb:revert-71372, r=petrochenkov

Revert #71372 ("Fix #! (shebang) stripping account space issue").

While #71372 fixed some of the problems `#!`-stripping had, it introduced others:
* inefficient implementation (`.chars().filter(...).collect()` on the entire input file)
  * this also means the length returned isn't always correct, leading to e.g. #71471
* it ignores whitespace anywhere, stripping ` # ! ...` which isn't a valid shebang
  * the definition of "whitespace" it uses includes newlines, which means even `\n#\n!\n...` is stripped as a shebang (and anything matching the regex `\s*#\s*!\s*`, and not followed by `[`, really)
* it's backward-incompatible but didn't go through Crater

Now, #71487 is already open and will solve all of these issues. But for running Crater, and just in case #71487 takes a bit longer, I decided it's safer to just revert #71372.

This will also make #71372's diff clearer, as it will start again from the original whitespace-unaware version.

r? @petrochenkov

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