[GlobalISel] Remove unused declaration applyCombineAnyExtTrunc

The corresponding function definition was removed by:

  commit a6be267
  Author: Jay Foad <[email protected]>
  Date:   Tue Feb 23 16:10:19 2021 +0000

Author: kazutakahirata

llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h

@@ -378,7 +378,6 @@ class CombinerHelper {
 
   /// Transform anyext(trunc(x)) to x.
   bool matchCombineAnyExtTrunc(MachineInstr &MI, Register &Reg);
-  void applyCombineAnyExtTrunc(MachineInstr &MI, Register &Reg);
 
   /// Transform zext(trunc(x)) to x.
   bool matchCombineZextTrunc(MachineInstr &MI, Register &Reg);