BUILD: debug: do not check the isolated_thread variable in non-threaded builds

BUILD: debug: do not check the isolated_thread variable in non-threaded builds

The build without thread support was broken by commit b30ced3d8 ("BUG/MINOR:
debug: fix incorrect profiling status reporting in show threads") because
it accesses the isolated_thread variable that is not defined when threads
are disabled. In fact both the test on harmless and this one make no sense
without threads, so let's comment out the block and mark the related
variables as unused.

This may have to be backported to 2.7 if the commit above is.