[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
```
diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibCxxMap.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibCxxMap.cpp
index 0929d49..96d9bcc 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibCxxMap.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibCxxMap.cpp
@@ -30,7 +30,6 @@
       : 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(
@@ -38,7 +37,6 @@
   }
 
   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 @@
   }
 
   ValueObjectSP parent() const {
-    static ConstString g_parent("__parent_");
     if (!m_entry_sp)
       return m_entry_sp;
     return m_entry_sp->GetSyntheticChildAtOffset(