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

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), 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

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

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

Author: lukekim

candle-flash-attn/src/ffi.rs

@@ -34,7 +34,6 @@ extern "C" {
         d: u32,
         d_rounded: u32,
         softmax_scale: f32,
-        softcap: f32,
 
         seqlen_q: u32,
         seqlen_k: u32,