Remove the concept of fragile reductions

They were a vestige of when Tree-sitter did sentential form-based
incremental parsing (as opposed to simply state matching). This was
elegant but not compatible with GLR as far as I could tell.

src/compiler/parse_table.h

@@ -43,7 +43,6 @@ struct ParseAction {
   int dynamic_precedence;
   rules::Associativity associativity;
   unsigned alias_sequence_id;
-  bool fragile;
   bool extra;
   bool repetition;
 };