Remove location info from `Exception#inspect`

Because location info (file name and line number) is kept in the backtrace,
it should not be kept in the result of `inspect` (and the exception object
itself), I think.

### Example

  ```ruby
  # example.rb
  begin
    raise "err"
  rescue => e
    p e
  end
  ```

#### Before this patch:

  ```
  $ bin/mruby example.rb
  example.rb:2: err (RuntimeError)
  ```

#### After this patch:

  ```
  $ bin/mruby example.rb
  err (RuntimeError)
  ```

1 files changed, 0 insertions, 1 deletions

diff --git a/src/string.c b/src/string.c
index 5a0a6a233..b3f07dfb0 100644
--- a/src/string.c
+++ b/src/string.c
@@ -1095,7 +1095,6 @@ mrb_str_equal_m(mrb_state *mrb, mrb_value str1)
   return mrb_bool_value(mrb_str_equal(mrb, str1, str2));
 }
 /* ---------------------------------- */
-mrb_value mrb_mod_to_s(mrb_state *mrb, mrb_value klass);
 
 MRB_API mrb_value
 mrb_str_to_str(mrb_state *mrb, mrb_value str)