Fix some unused, superfluous and uninitialized warnings

A few unused variable warnings were introduced by some previous patches. Since the variables were not part of a public API, they are removed in this MR.

In addition, this MR fixes a type sign mismatch when comparing ARRAY_SIZE() to int, superfluous braces to construct a vlc_chroma_description_t that wasn't updated to the chroma array removal.

Finally, some refactors on the yuvp chroma_filter are made for readability before adding a vlc_assert_unreachable() when the chroma doesn't have RGB packing in the case of an RGB_PACKED chroma.