UBI: more of clean-up terminology for self-checks

We have the "sefl-check" feature in UBI, but for historical reasons many
corresponding functions and commentaries in the code use term "paranoid check"
instead. Let's clean this up and use "self-check" everywhere.

This patch renames functions, amends messages and kills several redundant
debugging messages.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>