Removes unused bias accumulation flag

Simplifies backward parameters by removing an unused boolean that toggled bias accumulation.
Reduces maintenance surface and potential ABI issues; no behavior change expected.

Author: algo-home

csrc/flash_dmattn/src/flash.h

@@ -195,8 +195,6 @@ struct Flash_bwd_params : public Flash_fwd_params {
 
     bool deterministic;
     index_t dq_accum_split_stride;
-
-    bool accum_dbias;
 };
 
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