Fix redundant definitions of `NodeValue::getConst` (#7636)

Fix redundant definitions of `NodeValue::getConst` (#7636)

If multiple kinds have the same payload, cvc5 is currently generating
multiple copies of NodeValue::getConst() with the same template
argument, which results in a redefinition of the same function. This
commit modifies the mkmetakind script to avoid emitting redundant
definitions of NodeValue::getConst().