candle-flash-attn: remove duplicate softcap: f32 in run_mha FFI decl

[lukekim]

The Rust extern declaration of run_mha in candle-flash-attn/src/ffi.rs listed softcap: f32 twice — once between softmax_scale and seqlen_q, and again at the end of the parameter list (after window_size_right).

The C definition in candle-flash-attn/kernels/flash_api.cu only has a single softcap (at the end):

extern "C" void run_mha(
    ...
    float softmax_scale,
    uint32_t seqlen_q,
    ...
    int window_size_left,
    int window_size_right,
    float softcap
) { ... }

so the Rust-side signature declared 38 parameters while the call sites in src/lib.rs pass 37:

error[E0061]: this function takes 38 arguments but 37 arguments were supplied
   --> candle-flash-attn/src/lib.rs:169:13
error[E0061]: this function takes 38 arguments but 37 arguments were supplied
   --> candle-flash-attn/src/lib.rs:634:13

Drop the spurious copy between softmax_scale and seqlen_q so the Rust FFI matches the C ABI.

Validation

Reproduced while building spiced (spiceai/spiceai#10278) with --features cuda on CUDA 12.6 — the build now completes once this fix is applied.