fix: remove dead spill buffer path and invalid asm injection

[Bashmunta]

Removed an unreachable spill buffer path in generate_impl and deletes the corresponding add_buffer helper that injected a Rust variable declaration into the asm instruction stream. The path was dead because only num_limbs <= 6 are supported while MAX_REGS is 6; if ever activated, it would produce invalid inline assembly by embedding non-asm Rust code and by referencing a variable outside Rust scope via an asm operand, and moreover the buf operand was never used in the assembly template. Eliminating this code prevents a latent compile-time error, has no effect on current supported configurations, and clarifies that support for larger limb counts needs a proper prologue-based buffer setup and explicit usage in asm if added in the future.