Removed `String#try_convert` method from `mruby-string-ext` gem.

Because `try_convert` method rarely used in production. For mruby users,
we have `__to_str` utility method to check string type.

mrbgems/mruby-string-ext/mrblib/string.rb

 class String
 
-  ##
-  #  call-seq:
-  #     String.try_convert(obj) -> string or nil
-  #
-  # Try to convert <i>obj</i> into a String, using to_str method.
-  # Returns converted string or nil if <i>obj</i> cannot be converted
-  # for any reason.
-  #
-  #     String.try_convert("str")     #=> "str"
-  #     String.try_convert(/re/)      #=> nil
-  #
-  def self.try_convert(obj)
-    if self === obj
-      obj
-    else
-      nil
-    end
-  end
-
   ##
   # call-seq:
   #    string.clear    ->  string

mrbgems/mruby-string-ext/test/string.rb

@@ -4,13 +4,6 @@
 
 UTF8STRING = ("\343\201\202".size == 1)
 
-assert('String.try_convert') do
-  assert_nil String.try_convert(nil)
-  assert_nil String.try_convert(:foo)
-  assert_equal "", String.try_convert("")
-  assert_equal "1,2,3", String.try_convert("1,2,3")
-end
-
 assert('String#getbyte') do
   str1 = "hello"
   bytes1 = [104, 101, 108, 108, 111]