memtx: refactor `index_def_new`

Since `key_def_merge` sets the merged key definition's unique part count
equal to the new part count, the extra assignment in case the index is not
unique is redundant: remove it.

NO_CHANGELOG=<refactoring>
NO_DOC=<refactoring>
NO_TEST=<refactoring>

(cherry picked from commit 1d6c92e5)