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

var/spack/repos/builtin.mock/packages/multimethod/package.py

@@ -5,7 +5,7 @@
 
 from six import string_types
 
-import spack.architecture
+import spack.platforms
 from spack import *
 from spack.pkg.builtin.mock.multimethod_base import MultimethodBase
 
@@ -83,7 +83,7 @@ def has_a_default(self):
     #
     # Make sure we can switch methods on different target
     #
-    platform = spack.architecture.platform()
+    platform = spack.platforms.host()
     targets = list(platform.targets.values())
     if len(targets) > 1:
         targets = targets[:-1]