Remove unused import and update migration guide

Author: Vasileios Karakasis

docs/migration_2_to_3.rst

@@ -171,6 +171,9 @@ Other deprecations
 The :attr:`prebuild_cmd` and :attr:`postbuild_cmd` test attributes are replaced by the :attr:`prebuild_cmds` and :attr:`postbuild_cmds` respectively.
 Similarly, the :attr:`pre_run` and :attr:`post_run` test attributes are replaced by the :attr:`prerun_cmds` and :attr:`postrun_cmds` respectively.
 
+.. warning::
+   .. versionchanged:: 3.4
+      The :attr:`prebuild_cmd`, :attr:`postbuild_cmd`, :attr:`pre_run` and :attr:`post_run` attributes have been removed.
 
 
 Suppressing deprecation warnings

reframe/core/pipeline.py

@@ -32,7 +32,7 @@
 import reframe.utility.udeps as udeps
 from reframe.core.backends import (getlauncher, getscheduler)
 from reframe.core.buildsystems import BuildSystemField
-from reframe.core.containers import ContainerPlatform, ContainerPlatformField
+from reframe.core.containers import ContainerPlatformField
 from reframe.core.deferrable import _DeferredExpression
 from reframe.core.exceptions import (BuildError, DependencyError,
                                      PipelineError, SanityError,