Fixes for miRNA 4.0 arrays

The miRNA 4.0 arrays don't have MM probes, so I added a test to bypass creation of the mmfeature table. Also changed from using dbCreateIndicesPm(), which would automatically try to create an index for the mmfeature table (and fail), and try to index on the fid, which isn't unique for the miRNA 4.0 arrays.

This second change will affect indexing for all miRNA arrays, but they are small so it probably won't make a big performance hit.