[patch][gold] Remove empty Merge_map class

[Rafael Espíndola]

Ping. Lets focus on just this patch first. It is just deleting dead
code. To see it, noticed that:

* Merge_map is empty, so its methods (add_mapping, get_output_offset,
is_merge_section_for) don't use "this" and are really just free
standing functions. In this patch they are converted 1:1 into Relobj
methods since they all currently take a Relobj* as the first argument.

* The only use of a pointer to a Merge_map is in the comparison
"map->merge_map == merge_map" in
Object_merge_map::is_merge_section_for, which indicates that Merge_map
is now really just an ID.

* Merge_map is never allocated on its own, just as a member variable
of Eh_frame and Output_merge_base. Given that, we can just use a
pointer to Eh_frame/Output_merge_base and save the member. The common
base class of Eh_frame and Output_merge_base is Output_section_data,
so this patch s/Merge_map/Output_section_data/.

Cheers,
Rafael
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