Revert "use `rb_str2stm()` instead of `ID2SYM(rb_intern())`"

This reverts commit 0070c031ba8ad24f7df9eaabcb1b8623f6166cf6.

I've misunderstand here...
extconf: add rb_to_symbol func check

`rb_str2sym` is my misunderstand. Sorry... :(
rb-grn.h: add rb_to_symbol() backward compatibility macro
use rb_to_symbol instead of ID2SYM
suppress incompatible type conversion warning

warning: incompatible integer to pointer conversion
 assigning to 'const char *' from 'VALUE' (aka 'unsigned long')
  [-Wint-conversion]
  *name = rb_sym2str(rb_name);
use `SYM2STR` name for symbol GC macro

Using rb_sym2str name causes confusing.
There is differnce of original rb_sym2str and macro's rb_sym2str.
So, it shuold be rename.