[lldb][DataFormatter][NFC] Remove redundant variables in std::map formatter

Redundant since:
```
commit be3be28
Author: Enrico Granata <[email protected]>
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
```

Author: Michael137

lldb/source/Plugins/Language/CPlusPlus/LibCxxMap.cpp

@@ -30,15 +30,13 @@ class MapEntry {
       : m_entry_sp(entry ? entry->GetSP() : ValueObjectSP()) {}
 
   ValueObjectSP left() const {
-    static ConstString g_left("__left_");
     if (!m_entry_sp)
       return m_entry_sp;
     return m_entry_sp->GetSyntheticChildAtOffset(
         0, m_entry_sp->GetCompilerType(), true);
   }
 
   ValueObjectSP right() const {
-    static ConstString g_right("__right_");
     if (!m_entry_sp)
       return m_entry_sp;
     return m_entry_sp->GetSyntheticChildAtOffset(
@@ -47,7 +45,6 @@ class MapEntry {
   }
 
   ValueObjectSP parent() const {
-    static ConstString g_parent("__parent_");
     if (!m_entry_sp)
       return m_entry_sp;
     return m_entry_sp->GetSyntheticChildAtOffset(