[lld] [ELF] [MIPS] Remove duplicate logic

Summary: With r218633, the logic that monitors which shared library symbols were used was copied from the MIPS lld backend to ELF classes, making it available to all ELF backends. However, this made the isDynSymEntryRequired() functions in MipsDynamicLibraryWriter.h/MipsELFWriters.h/MipsExecutableWriter.h to be duplicated logic, since this is already implemented in OutputELFWriter<>/DefaultLayout. This patch removes this duplicated code from MIPS.

Reviewers: Bigcheese, shankarke

Reviewed By: shankarke

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5564