GitClient: Replace describe command with QtcProcess

There is no need to use VcsCommand when NoOutput is passed.
Get rid of execBgCommand().

Change-Id: I58354e99ddc0c4049325560022ba6e755092b817
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/vcsbase/vcsbaseclient.h

@@ -59,11 +59,6 @@ public:
                               VcsBaseEditorWidget *editor = nullptr,
                               JobOutputBindMode mode = NoOutputBind) const;
 
-    VcsCommand *execBgCommand(const Utils::FilePath &workingDirectory,
-                              const QStringList &args,
-                              const std::function<void (const QString &)> &outputCallback,
-                              unsigned flags = 0) const;
-
     void enqueueJob(VcsCommand *cmd, const QStringList &args,
                     const Utils::ExitCodeInterpreter &interpreter = {}) const;