configure: drop check for default xapian backend
Starting from xapian 1.3.5, xapian switched default backend to glass.
From
00cdfe10 (build: drop support for xapian versions less than 1.4,
2020-04-22), we only support xapian 1.4.0+. Effectively, we don't need
to check for default xapian backend anymore.
Further more, from
99a7aac8 (test: drop use of db_ending, 2020-07-29),
our test framework has become independence from default xapian.
Let's drop it.
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>