gdb: Convert language_data::la_varobj_ops to a method

Convert language_data::la_varobj_ops member variable to a virtual
method language_defn::varobj_ops.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_language_data): Remove la_varobj_ops
	initializer.
	(ada_language::varobj_ops): New member function.
	* c-lang.c (c_language_data): Remove la_varobj_ops
	initializer.
	(cplus_language_data): Likewise.
	(cplus_language::varobj_ops): New member function.
	(asm_language_data): Remove la_varobj_ops initializer.
	(minimal_language_data): Likewise.
	* d-lang.c (d_language_data): Likewise.
	* f-lang.c (f_language_data): Likewise.
	* go-lang.c (go_language_data): Likewise.
	* language.c (language_defn::varobj_ops): New function.
	(unknown_language_data): Remove la_varobj_ops
	initializer.
	(auto_language_data): Likewise.
	* language.h (language_data): Remove la_varobj_ops field.
	(language_defn::varobj_ops): Declare new member function.
	* m2-lang.c (m2_language_data): Remove la_varobj_ops initializer.
	* objc-lang.c (objc_language_data): Likewise.
	* opencl-lang.c (opencl_language_data): Likewise.
	* p-lang.c (pascal_language_data): Likewise.
	* rust-lang.c (rust_language_data): Likewise.
	* varobj.c (varobj_create): Update call to varobj_ops.
	* varobj.h (default_varobj_ops): Delete define.