Remove varobj_language_string, languages and varobj_languages

Remove varobj_language_string, languages and varobj_languages

This patch does some cleanups, removing some language-related stuff.
Note that mi_cmd_var_info_expression uses varobj_language_string,
which is redundant, because we can get language name from
lang->la_natural_name.

varobj_language_string doesn't have "Ada", which looks like a bug to
me.  With this patch applied, this problem doesn't exist, because the
language name is got from the same place (field la_natural_name).

gdb:

2013-11-07  Yao Qi  <yao@codesourcery.com>

	* mi/mi-cmd-var.c: Include "language.h".
	(mi_cmd_var_info_expression): Get language name from
	language_defn.
	* varobj.c (varobj_language_string): Remove.
	(variable_language): Remove declaration.
	(languages): Remove.
	(varobj_get_language): Change the type of return value.
	(variable_language): Remove.
	* varobj.h (enum varobj_languages): Remove.
	(varobj_language_string): Remove declaration.
	(varobj_get_language): Update declaration.

gdb/doc:

2013-11-07  Yao Qi  <yao@codesourcery.com>

	* gdb.texinfo (GDB/MI Variable Objects): Update doc about the
	output of "-var-info-expression".