Centralize retrieving the "parent" build configuration

If a step is part of a build configuration, that is used, but many steps
can be used as part of a deploy configuration.
In that case the active build configuration of the step's target must be
used, and that logic was duplicated many times.
Instead, let BuildStep::buildConfiguration take care of that logic. For
steps that are not offered for deploy configurations there is no
semantic difference, and for the others this removes code duplication.

Change-Id: I02f3bb50226590092cedcec02fce6fde9c7c6e63
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildstep.h

@@ -55,7 +55,6 @@ public:
     explicit CMakeBuildStep(ProjectExplorer::BuildStepList *bsl);
 
     CMakeBuildConfiguration *cmakeBuildConfiguration() const;
-    CMakeBuildConfiguration *targetsActiveBuildConfiguration() const;
 
     bool init(QList<const BuildStep *> &earlierSteps) override;
     void run(QFutureInterface<bool> &fi) override;