[ELF] - Remove unused variable. NFC.

It triggered a bot failture somehow today:
http://lab.llvm.org:8011/builders/lld-perf-testsuite/builds/7993

llvm-svn: 344344

lld/ELF/SyntheticSections.h

@@ -307,8 +307,6 @@ private:
 
   uint64_t Size = 0;
 
-  size_t LocalEntriesNum = 0;
-
   // Symbol and addend.
   typedef std::pair<Symbol *, int64_t> GotEntry;