WikiExporter: Remove unnecessary check for SCHEMA_COMPAT_WRITE_OLD flag

WikiExporter: Remove unnecessary check for SCHEMA_COMPAT_WRITE_OLD flag

WikiExporter used to require SCHEMA_COMPAT_WRITE_OLD to be enabled,
until that requirement was fixed in I5ea972bb07ca1cfb3a2ad8ef120aef7.
However, I failed to remove the explicit check for the flag at the
time, causing all exports to fail in SCHEMA_COMPAT_NEW mode. This
change removes the obsolete check.

Bug: T236735
Change-Id: I809ed4e2f1f30fdc4bd817f815d733d8a62f3d4f
(cherry picked from commit d9209707cc62ea2eb0f0fe9d2c79e56a8cc87552)