Removed check for ambiguous return values for lens lookups.

It can *never* be guaranteed that the return list of lenses of lensfun contains only one entry, even if the lens model name was exactly correct.  Lenses with a longer lens name (containing the shorter one) will also be returned.  However, it is guaranteed that the exact match will be returned as the zeroth list element.

Example to test: The 18-135mm Canon EF-S lens comes in two variants, one with STM and one without.  The short form is not auto-detected without this fix.  See e.g. the CR2 file linked in http://www.darktable.org/redmine/issues/9812