Remove docid_word_positions_db + fix deletion bug

That would happen when a word was deleted from all exact attributes
but not all regular attributes.

milli/src/update/index_documents/helpers/mod.rs

@@ -14,8 +14,8 @@ pub use grenad_helpers::{
 };
 pub use merge_functions::{
     concat_u32s_array, keep_first, keep_latest_obkv, merge_cbo_roaring_bitmaps,
-    merge_obkvs_and_operations, merge_roaring_bitmaps, merge_two_obkvs,
-    roaring_bitmap_from_u32s_array, serialize_roaring_bitmap, MergeFn,
+    merge_obkvs_and_operations, merge_roaring_bitmaps, merge_two_obkvs, serialize_roaring_bitmap,
+    MergeFn,
 };
 
 use crate::MAX_WORD_LENGTH;