[PM] Remove a dead field.

This was dead even before I refactored how we initialized it, but my
refactoring made it trivially dead and it is now caught by a Clang
warning. This fixes the warning and should clean up the -Werror bot
failures (sorry!).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226376 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed