remove convertNgrams stub from dictionary header
remove convertNgrams stub from dictionary header

Summary: This diff removes a line of dead code.

Reviewed By: EdouardGrave

Differential Revision: D5031495

fbshipit-source-id: 8e200a9a4e5fae9ec5dc4af2c17fac185a719310

facebookresearch/fastText,facebookresearch/fastText,facebookresearch/fastText,facebookresearch/fastText,facebookresearch/fastText,facebookresearch/fastText