completion: Place the completion helper in libexec

completion: Place the completion helper in libexec

This patch reorganize the bash completion scripts in order to install
the binary helper (gst-completion-helper) in libexec path rather then
share folder. Most Linux hierarchy compliance requires that no binary
executable are placed in share. We also cleanup the unused .pc entries
and remove copy pasted parts of the script. Note that other project
including the common helper, should now use $_GST_HELPER to read
the binary executable gst-completion-helper. This helper is not longer
version, as it is placed in a versionned subfolder
(libexec/gstreamer.10) just like the other helpers (scanner and ptp).