Get rid of ARM_ELF_SYM_IS_THUMB flag.

Summary:
This was used with the old ARM vs. Thumb detection code but is not
required anymore.

Reviewers: tberghammer, clayborg

Subscribers: fjricci, aemerson, lldb-commits, rengolin

Differential Revision: http://reviews.llvm.org/D16099

llvm-svn: 257429