LOAD.. code for MM has some suspect/dead code

Discovered in HIVE-18571 and added TODO-s that need to be addressed.
E.g.

if (isMmTableWrite) {
           // We will load into MM directory, and delete from the parent if needed.
          // TODO: this looks invalid after ACID integration. What about base dirs?
           destPath = new Path(destPath, AcidUtils.deltaSubdir(writeId, writeId, stmtId));
...
         // TODO: loadFileType for MM table will no longer be REPLACE_ALL
           filter = (loadFileType == LoadFileType.REPLACE_ALL)

2 places like that

Also replaceFiles has isMmTableWrite flag that should no longer be needed (since for a transactional table we should never replace files). Either there's some invalid code path that relies on it (load table?), or it is just unused and needs to be removed.
Also used in 2 places, "TODO: this should never run for MM tables anymore. Remove the flag, and maybe the filter?"