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>

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