[ELF] Delete an unused special rule from isStaticLinkTimeConstant. NFC

Weak undefined symbols are preemptible after D71794.

  if (sym.isPreemptible)
    return false;
  if (!config->isPic)
    return true;
  // isPic means includeInDynsym is true after D71794.

  ...

  // We can delete this if because it can never be true.
  if (sym.isUndefWeak)
    return true;

Differential Revision: https://reviews.llvm.org/D71795