compiletest: remove `pretty-expanded` directive and infra

Foreword
========

Let us begin the journey to rediscover what the `//@ pretty-expanded`
directive does, brave traveller --

    "My good friend, [..] when I wrote that passage, God and I knew what
    it meant. It is possible that God knows it still; but as for me, I
    have totally forgotten."

                                 -- Johann Paul Friedrich Richter, 1826

We must retrace the steps of those before us, for history shall guide us
in the present and inform us of the future.

The Past
========

Originally there was some effort to introduce more test coverage for `-Z
unpretty=expanded` (in 2015 this was called `--pretty=expanded`). In
[Make it an error to not declare used features #23598][pr-23598], there
was a flip from `//@ no-pretty-expanded` (opt-out of `-Z
unpretty=expanded` test) to `//@ pretty-expanded` (opt-in to `-Z
unpretty=expanded` test). This was needed because back then the
dedicated `tests/pretty` ("pretty") test suite did not existed, and the
pretty tests were grouped together under `run-pass` tests (I believe
`ui` test suite didn't exist back then either). Unfortunately, in this
process the replacement `//@ pretty-expanded` directives contained a
`FIXME #23616` linking to [There are very few tests for `-Z unpretty`
expansion #23616][issue-23616]. But this was arguably backwards and
somewhat misleading, as noted in
<https://github.com/rust-lang/rust/issues/23616#issuecomment-484999901>:

    The attribute is off by default and things just work if you don't
    test it, people have not been adding the `pretty-expanded`
    annotation to new tests even if it would work.

Which basically renders this useless.

The Present
===========

As of Nov 2024, we have a dedicated `pretty` test suite, and some time
over the years the split between `run-pass` into `ui` and `pretty` test
suites caused all of the `//@ pretty-expanded` in `ui` tests to do
absolutely nothing -- the compiletest logic for `pretty-expanded` only
triggered in the *pretty* test suite, but none of the pretty tests use
it. Oops.

The Future
==========

Nobody remembers this, nobody uses this, it's misleading in ui tests.
Let's get rid of this directive altogether.

[pr-23598]: https://github.com/rust-lang/rust/pull/23598
[issue-23616]: https://github.com/rust-lang/rust/issues/23616

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