[lldb/dotest] Remove the "xunit" result formatter
My understanding is that this was added to make dotest interact well
with the GreenDragon bots, back when dotest was the main test driver.
Now that everything goes through lit (which has its own xunit
formatter), it seems largely irrelevant.
There are more cleanups that can be done after removing this be done
here, but this should be enough to test the waters.
Differential Revision: https://reviews.llvm.org/D83545