Remove dead code from SymbolFileDWARF:

lldb::TypeSP
SymbolFileDWARF::FindDefinitionTypeForDIE (
    DWARFCompileUnit* dwarf_cu,
    const DWARFDebugInfoEntry *die,
    const lldb_private::ConstString &type_name);

This function isn't used as it has been replaced by:

lldb::TypeSP
SymbolFileDWARF::FindDefinitionTypeForDWARFDeclContext (
    const DWARFDeclContext &die_decl_ctx);

I am about to change the way we resolve C/C++ class/struct/union types and want to clean up SymbolFileDWARF before I start.