Only ELF has a dynamic symbol table. Remove it from ObjectFile.

COFF has only one symbol table.
MachO has a LC_DYSYMTAB, but that is not a symbol table, just extra info about
the one symbol table (LC_SYMTAB).
IR (coming soon) also has only one table.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200488 91177308-0d34-0410-b5e6-96231b3b80d8