build: remove rpm spec file and build rules

build: remove rpm spec file and build rules

As discussed in issue #2204 this patch removes the dracut.spec file from
the repository.  The advantage of this patch is that it creates a
dracut-version.tar.xv file that can be more easily consumed by the
downstream distributions because there's no rpm spec file included in
the distribution.

Tested with a downstream rpm spec fiie:

```
cd dracut
VERSION=`git describe --abbrev=0 --tags --always`
make clean
make dist
cp dracut-${VERSION}.tar.xz ../
cd ..

xz -d -v dracut-${VERSION}.tar.xz
tar rf dracut-${VERSION}.tar dracut.spec
xz -9 dracut-${VERSION}.tar

rm -rf rmpbuild
mkdir -p rpmbuild/{BUILD,BUILDROOT,RPMS,SOURCES,SPECS,SRPMS}
rpmbuild -ta --define "_topdir `pwd`/rpmbuild" -v dracut-${VERSION}.tar.xz
```

Fixes issue #2204