This reverts commit 15e77a56b7.
Subpriorities were added in https://github.com/neovim/neovim/pull/27131
as a mechanism for enforcing query order when using iter_matches in the
Tree-sitter highlighter. However, iter_matches proved to have too many
complications to use in the highlighter so we eventually reverted back
to using iter_captures (https://github.com/neovim/neovim/pull/27901).
Thus, subpriorities are no longer needed and can be removed.