This reverts commit 2c47dddbc1.
Now, `spack compiler` writes by default in packages.yaml. Entries
in old `compilers.yaml` are converted to external specs as a way to
support legacy configuration.
Since this operation is expensive, an environment variable can be
used to enforce the deprecation of `compiler.yaml`.
The --mixed-toolchain option has been deprecated, since it stops to
make sense once compiler are treated as nodes.
0705a5f468