ProjectExplorer: Remove Run{Control,Configuration}::abi

Unused now, and arguably not a valid concept anymore.
A run control may consist of different runworkers,
'operating' on different systems.

Since the removed RunConfiguration::abi() implementation uses
only public Target ABI, user code could revive that locally
in case it was needed again.

Change-Id: I658c6e88b4a1aec18bf4de91fc86230552bc0710
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/qmlprojectmanager/qmlprojectrunconfiguration.h

@@ -44,7 +44,6 @@ public:
 private:
     ProjectExplorer::Runnable runnable() const final;
     QString disabledReason() const final;
-    ProjectExplorer::Abi abi() const final;
     void updateEnabledState() final;
 
     QString mainScript() const;