etc.c: small cleanup (delete unused functions)

mrb_data_object_alloc: unnecessary cast
mrb_lastline_get: mruby doesn't support $_, weird and unused code
mrb_exec_recursive: see 4e46abb86914b36e70b5f3ad0826d0b648e9b4ef
mrb_block_proc: unused, doesn't really do anything (or needs to be
rewritten)
mrb_obj_to_sym: actually use id (and MSVC detected unreachable code)
Merge pull request #1668 from cremno/etc-c-cleanup

etc.c: small cleanup