See <http://lists.manyfish.co.uk/pipermail/neon/2013-February/001533.html> "Re:
About ne_set_read_timeout" for why ne_close_connection is not a solution here
(and currently would not work anyway, for the thread calling abort would block
on the aGlobalNeonMutex introduced with 510da291
"forced to make certain neon api calls thread-safe due to cups" until the
blocking thread was done with its neon call) and "a new, dedicated, thread-safe
ne_session_* API call will be needed to allow aborting of a running request."

So enable DAVResourceAccess::abort again and instead disable the underlying
NeonSession::abort.  i#106766 "Crash when cancelling extension update" does not
give any good rationale why DAVResourceAccess::abort had to be disabled, so it
might well have been because the underlying NeonSession::abort was bogus to
begin with.

Change-Id: If8293b93a960907726208bb2f93c375d83357ed3