exif: always try to use Exiv2's lens detection for Olympus

Example:
Exif.OlympusEq.LensType                      Byte        6  Olympus Zuiko Digital ED 14-42mm F3.5-5.6
Exif.OlympusEq.LensModel                     Ascii      32  OLYMPUS 14-42mm Lens

The lensfun-0.3.0 database has been cleaned up and has proper matches to LensType
The lensfun-0.2.8 database has something that looks like LensModel, but not quite.

LensModel is very ambiguous in any case and thus probably shouldn't be used in any case.

Particularly when taking in account metadata is read at import time, it makes a lot of sense
to use the most "future-proof" option, which will be LensType.

Relevant commits:
eee664223f7eff916b3f6a6d92a6d2f305c9d097
e710949c915a0f2476d4cb3fbdea9e90af69d074
09bac2436d5cbbc13e71ad8649394bada005a53f