ELF: Remove field for .gdb_index in InStruct. NFC.

Summary: This field is unreferenced outside of createSyntheticSections.

Reviewers: ruiu, pcc, espindola, grimar

Reviewed By: grimar

Subscribers: grimar, emaste, arichardson, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58423

llvm-svn: 354449