Inline ntt.c

[mkannwischer]

#735 hoisted out the C portion of poly_ntt into poly_ntt_c resulting in now three levels of indirection poly_ntt -> poly_ntt_c -> mld_ntt. Similar for the invntt.
We should remove that last level of indirection and simply inline mld_ntt into poly_ntt_c (similar for invntt). Consequently, the ntt.c/h files will no longer be needed. This also aligns with mlkem-native.