[lldb][DataFormatter][NFC] Remove redundant variables in std::map formatter
Redundant since:
```
commit be3be28b5d5c97de1c26bf069e0b82043d938f30
Author: Enrico Granata <egranata@apple.com>
Date: Mon Oct 3 23:33:00 2016 +0000
Changes to the std::multimap formatter to make it work against trunk libc++
Fixes rdar://28237486
llvm-svn: 283160
```