AMDGPU/GlobalISel: Fix attempt to select non-legal instr in mir test

Delete inst-select-insert.xfail.mir.
G_INSERT instructions in inst-select-insert.xfail.mir are no longer
legal after D114198. This breaks build bots, since builds with
LLVM_ENABLE_ASSERTIONS=Off don't check for legality and report cannot
select while build with LLVM_ENABLE_ASSERTIONS=On reports instruction
is not legal.