Removed PHAR builds support

This also removes all tooling around PHAR builds.

While PHARs have some use for some community members, PHARs:

 * do not come with a runtime/environment
 * are inherently unsafe, because (even if signed) you are relying on packages downloaded
   and installed by maintainer who created the `.phar` file
 * do not match your system requirements, which may be misaligned with bundled libraries,
   since you are relying on the maintainer's ( @Roave / @Ocramius ) selected set of
   dependencies

There are various ways to install PHP tools that are way better than PHAR, and
that are to be endorsed instead.

## `nix` flake

See https://nixos.wiki/wiki/flakes

Nix flakes are completely immutable, stable, fully reproducible, customizable,
and come with a full supply chain traceability of all dependencies.

For maximum stability in your builds, use a nix flake. Patches are welcome to
turn this package into a nix flake in itself.

## `composer.json`, `composer.lock` and `composer install`

If you have an environment, and you want to install this package into it, generate
a `composer.json` and `composer.lock` at a specific location:

```sh
cd path/to/tools
composer require roave/backward-compatibility-check
git add composer.json
git add composer.lock
git commit -m "Managing own locked version of \`roave/backward-compatibility-check\`"
```

You can then `composer install` from that directory at any time, and
run `./path/to/tools/vendor/bin/roave-backward-compatibility-check`.

`composer install` will verify that your environment is suited for installation.

## use a `Dockerfile`

This approach comes with the same downsides of `.phar` files, but:

* tooling to scan for vulnerabilities inside docker images exist, and is
  very much growing
* a docker image comes with the runtime to execute `roave/backward-compatibility-check`,
  and you can even run the full test suite before baking the image and calling
  it "compatible"

.gitattributes

@@ -4,7 +4,6 @@
 /.github/ export-ignore
 /.gitignore export-ignore
 /box.json.dist export-ignore
-/build-phar.sh export-ignore
 /infection.json.dist export-ignore
 /phpcs.xml.dist export-ignore
 /phpunit.xml.dist export-ignore

.gitignore

@@ -1,7 +1,6 @@
 /vendor
 /infection-log.txt
 /box.json
-/box.phar
 /dist
 phpstan.neon
 phpunit.xml