lib/crypto: x86/blake2s: Drop check for nblocks == 0

Since blake2s_compress() is always passed nblocks != 0, remove the
unnecessary check for nblocks == 0 from blake2s_compress_ssse3().

Note that this makes it consistent with blake2s_compress_avx512() in the
same file as well as the arm32 blake2s_compress().

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251102234209.62133-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>