AccessFlags factories do not require necessary arguments

AccessFlags is an object modeling access flag as a classfile element. It is essentially an integer bit mask, bound with a location to allow specific parsing.

However, this bit mask is version specific too. For example, pre-valhalla class files may optionally have the ACC_SUPER bit set for no impact; but once valhalla arrives, it will become ACC_IDENTITY where non-set bit would mean value classes. However, we still want to interpret pre-valhalla classes without this bit set as identity.

As a result, the current `AccessFlags.ofClass` `AccessFlags.ofMethod` `AccessFlags.ofField` methods become problematic if we want to support version-specific parsing, as it's not always proper to assume a latest class file version.

The proposed action is to remove the 6 `ofXxx` factory methods, making `AccessFlags` more similar to bytecode `Label` or `BootstrapMethodsAttribute`, that the creation will be through `withFlags` methods on respective builders. These builders can capture ClassFile versions from context, and thus correctly validate/parse/interpret access flags, which we can enhance in the future.