XML: upgrade-2.10.xsl: simplify thanks to diehard assumptions wrt. input

Such assumptions were expected since beginning, since there were
plentiful cases where the processing would break in one way or another.

For instance:

• many of {instance,meta}_attributes would only be propagated to the result of upgrade-2.10.xsl if (but not necessarily only if) they contained at least a single nvpair originally -- the child that is mutually exclusive with @id-ref attribute at the parent, as imposed with the schema
• referential integrity could be broken easily if some wild use (or abuse) of @id-ref based synctactical simpliciation; an example of this is the new test2to3{enter,leave}/021 test case (hint: compare the result of test-2-leave part with the direct ouput when transforming test-2-enter input with upgrade-2.10.xsl)

So hopefully by now it clear how furtunate was to set the expectations
about the raw upgrade material firmly with the unfold-transform-refold
logic established in several past commits.