Remove ada-varobj.h.

As a result of previous patch, extern functions in ada-varobj.c can be
made static, and ada-varobj.h can be removed too.

gdb:

2013-10-17  Yao Qi  <yao@codesourcery.com>

        * Makefile.in (HFILES_NO_SRCDIR): Remove ada-varobj.h.
        * ada-varobj.c: Remove the include of ada-varobj.h.
        (ada_varobj_get_number_of_children): Declare.
        (ada_varobj_get_name_of_child): Make it static.
        (ada_varobj_get_path_expr_of_child): Likewise.
        (ada_varobj_get_value_of_child): Likewise.
        (ada_varobj_get_type_of_child): Likewise.
        (ada_varobj_get_value_of_array_variable): Likewise.
        * ada-varobj.h: Remove.