Remove configuration of XSLT version in Saxon

[rdeltour]

Setting the "XSLT_VERSION" feature on Saxon's TransformerFactory issued a warning on recent Saxon version (9.8+).

According to @michaelkay:

At one time (before 3.0 was finalised) Saxon had the option to operate
as either a 2.0 or a 3.0 conformant processor. If you selected 2.0 (in
the API), then we tried hard to suppress all 3.0 features such as new
functions, xsl:iterate, etc. Simply setting version="2.0" in the
stylesheet doesn't have this effect (3.0 features will still work, so
you don't pass the conformance tests for 2.0). At some later date we
decided we would no longer have a 2.0-conformant mode: if you request
2.0, we fall back to 3.0, so you get the 3.0 features whether you like
it or not. We made this decision because the level of backwards
compatibility between 2.0 and 3.0 is very high, so pretty well all
non-error stylesheets will run unchanged.

This commit removes the code setting the "XSLT_VERSION" feature.

Fix #277.

[ndw]

👍