Avoid introducing certain lexical conflicts during parse state merging

The current pretty conservative approach is to avoid merging parse states which
would cause a pair tokens to co-exist for the first time in any parse state,
where the two tokens can start with the same character and at least one of the
tokens can contain a character which is part of the grammar's separators.

src/compiler/parse_table.h

@@ -96,8 +96,6 @@ class ParseTable {
 
   std::vector<ParseState> states;
   std::map<rules::Symbol, ParseTableSymbolMetadata> symbols;
-
-  std::set<rules::Symbol> mergeable_symbols;
 };
 
 }  // namespace tree_sitter