Remove symbol hash table from mrb_state structure.

Use linear search instead. Number of symbols is usually small (<1K), so
we don't need performance boost from hash tables. In our benchmark
measurement, hash tables consumes 790KB for `build/full-debug/mrbtest`.

include/mruby.h

@@ -239,7 +239,6 @@ typedef struct mrb_state {
 #endif
 
   mrb_sym symidx;
-  struct kh_n2s *name2sym;      /* symbol hash */
   struct symbol_name *symtbl;   /* symbol table */
   size_t symcapa;