Remove a discarded comparison.

mrb_method_search() does not return NULL.
Instead it raises an exception.
So it is not necessary to evaluate
a return value of mrb_method_search().
Unify include guard styles and header comments.
Merge pull request #2550 from cubicdaiya/issues/unify_include_guards

Unify include guard styles and header comments.
Merge pull request #2552 from cubicdaiya/issues/eval_return_value_mrb_method_search

Remove a discarded comparison.