[lldb] Eliminated unused local variable

I got misled by this remnant from earlier changes.

Author: amccarth-google

lldb/source/Commands/CommandObjectTarget.cpp

@@ -4332,7 +4332,6 @@ class CommandObjectTargetSymbolsAdd : public CommandObjectParsed {
                 module_spec.GetSymbolFileSpec() = symfile_spec;
             }
 
-            ArchSpec arch;
             bool symfile_exists =
                 FileSystem::Instance().Exists(module_spec.GetSymbolFileSpec());