Revert "Make the linker API thread-safe"

This reverts commit b5e8b3b1.

I reverted it because one of these two patches
  9e6e4796 Add purgeObj()
  b5e8b3b1 Make the linker API thread-safe

causes a seg-fault on Windows.  The seg-fault happens immediately
the linker is invoked, in ghci or in Template Haskell.

I believe that it is the "linker API thread-safe" commit that causes
the seg-fault; it happens even if the "purgeObj" commit alone is
reverted.  But since the two patches mess with the same code, to
revert the "linker API" patch I had revert both.