CMakeBuildSystem: Remove some left-overs

From removing the "first build in temporary directory" mechanics.

The method that would previously switch the build directory from
temporary to real now just ensures that the build directory exists and
passes the input build directory through. Name it encordingly and remove
the confusing return value.

Remove condition that now was always true.

Amends 0e613918b6

Change-Id: I8d4cff23b3fcba895bc7338e7277d44efb46e8e7
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildsystem.h

@@ -191,7 +191,7 @@ private:
 
     void wireUpConnections();
 
-    Utils::FilePath buildDirectory(const BuildDirParameters &parameters);
+    void ensureBuildDirectory(const BuildDirParameters &parameters);
     void stopParsingAndClearState();
     void becameDirty();