Remove the spack.architecture module (#25986)

The `spack.architecture` module contains an `Arch` class that is very similar to `spack.spec.ArchSpec` but points to platform, operating system and target objects rather than "names". There's a TODO in the class since 2016:

abb0f6e27c/lib/spack/spack/architecture.py (L70-L75)

and this PR basically addresses that. Since there are just a few places where the `Arch` class was used, here we query the relevant platform objects where they are needed directly from `spack.platforms`. This permits to clean the code from vestigial logic.

Modifications:
- [x] Remove the `spack.architecture` module and replace its use by `spack.platforms`
- [x] Remove unneeded tests

lib/spack/docs/developer_guide.rst

@@ -210,15 +210,6 @@ Spec-related modules
      but compilers aren't fully integrated with the build process
      yet.
 
-:mod:`spack.architecture`
-  :func:`architecture.default_arch <spack.architecture.default_arch>` is used
-  to determine the host architecture while building.
-
-  .. warning::
-
-     Not yet implemented.  Should eventually have architecture
-     descriptions for cross-compiling.
-
 ^^^^^^^^^^^^^^^^^
 Build environment
 ^^^^^^^^^^^^^^^^^