remove error highlighting for tree-sitter-elixir

This will become more important with the HEEx grammar being added.
Error highlighting with the Elixir grammar is a bit jumpy because
in some scenarios, a bit of missing syntax can force tree-sitter to
give up on error recovery and mark the entire tree as an error.
This ends up looking bad when editing. We don't typically highlight
error nodes so I'm inclined to leave it out of the highlights here.

runtime/queries/elixir/highlights.scm

@@ -217,5 +217,3 @@
   "<<"
   ">>"
 ] @punctuation.bracket
-
-(ERROR) @warning