dt-bindings: Drop DT_MK_SCHEMA_FLAGS conditional selecting schema files

Since commit ef8795f3 ("dt-bindings: kbuild: Use DTB files for
validation"), dt-mk-schema always needs a complete list of schemas, so
the conditional using DT_MK_SCHEMA_FLAGS should be removed.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220727211100.3249417-1-robh@kernel.org