In generics code, get rid of checks for ClassType

[msridhar]

There are various places in GenericsChecks that we only perform checks for ClassTypes, e.g.:

NullAway/nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java

Line 304 in 4b6e672

if (lhsType instanceof Type.ClassType && rhsType instanceof Type.ClassType) {

NullAway/nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java

Lines 336 to 338 in 4b6e672

	if (formalReturnType instanceof Type.ClassType
	&& returnExpressionType instanceof Type.ClassType) {
	boolean isReturnTypeValid =

I don't think these checks are needed, and in fact we may miss issues (e.g., if both types are array types and inside there are nested type arguments with mismatched nullability). We should remove these checks and add corresponding tests.