[AArch64][GISel] Drop custom selectors for ptrauth_* intrinsics (#75328)

Drop custom selector code for ptrauth_(sign|strip|blend) intrinsics from
AArch64InstructionSelector::selectIntrinsic function.

The code for strip and blend intrinsics was needed because of a bug in
TableGen fixed in 78623b07. The
ptrauth_sign intrinsic was presumably fixed long ago.