Interaction of the Adenine-Thymine Watson-Crick and Adenine-Adenine Reverse-Hoogsteen DNA Base Pairs with Hydrated Group IIa (Mg2+, Ca2+, Sr2+, Ba2+) and IIb (Zn2+, Cd2+, Hg2+) Metal Cations: Absence of the Base Pair Stabilization by Metal-Induced Polarization Effects