[InstCombine] Remove unused function createInstructionCombiningPass

The last use was removed by:

  commit 934c82d3
  Author: Florian Hahn <flo@fhahn.com>
  Date:   Fri Feb 24 13:39:32 2023 +0100

Once I remove createInstructionCombiningPass, then:

InstructionCombiningPass::InstructionCombiningPass(unsigned MaxIterations)

becomes unused.  Once I remove that:

InstructionCombiningPass::MaxIterations is always initialized with
InstCombineDefaultMaxIterations, so this patch does the constant
propagation and removes InstructionCombiningPass::MaxIterations as
well.

Differential Revision: https://reviews.llvm.org/D152641