Replace `_value_equality_values_cls_` in Phased* gates via specialized cirq.GateFamily's.

[tanujkhattar]

Currently, only three gates: PauliString, PhasedXPowGate and PhasedISwapPowGate override the _value_equality_values_cls_ method to return different class types for comparison based on the phase exponent / number of elements in the pauli string.

For phased* gates specifically, This is not ideal and causes confusion because of multiple reasons:

1. Equality across types is a complex problem is not consistent across Cirq. There are numerous other pairs of gates which are compatible which each other but do not currently support equality across types. For example: cirq.FSimGate and cirq.ISwapPowGate (and other related pairs); cirq.PhasedXZGate and pauli gates; etc.
2. With the advent of cirq.GateFamily class; equality across types is supposed to be handled by specialized gate families which understand the different types. For example, I'm currently writing a cirq_google.FSimGateFamily which can be used to check and convert types between any fsim compatible gate (i.e. fsim, phasedfsim, czpow, iswappow and phasediswap).

Therefore, I propose that we remove equality across types from PhasedXPowGate and PhasedISwapPowGate and use specialized gate families where such cross type equality checks are needed.

cc #3243

[tanujkhattar]

We should do this.

• Think about how to deprecate reasonably.
• We should only yell whenever it does comparison across types.