Remove long-form plate arguments.

This brings the code into line with the document changes in https://github.com/openmicroscopy/ome-documentation/pull/708 in response to the forum post http://www.openmicroscopy.org/community/viewtopic.php?f=5&t=7472&p=13743#p13736

To test this PR check that the `-n` and `-x` options work for both images and plates for both `bin/omero import` and `import-cli`. Because of the changes the other long form arguments, especially the in-place import arguments, should be tested to check that there has been no knock-on effect of removing the two options in this PR.