Revert "[GISel] Add LookThroughInstrs for getIConstantVRegVal and getIConstan… (#68327)"

This reverts commit 28ae42e6.

The assert in getIConstantVRegVal was not updated for this change.
The ValAndVReg->VReg == VReg check fails if any look through happens.

RISC-V was the only target using the lookthrough functionality, but I'm
not sure it was needed so I'm removing that too.