Consider removing HasFoo/ClearFoo() for proto2 message fields in C#

[jskeet]

To clarify that title, by "proto2 message fields" I mean "fields in a proto2-syntax message, where the type of the field is a message type".

This code:

var person = GetPersonFromSomewhere();
if (person.HasAddress)
{
   // Do something with the address field
}

is just equivalent to:

var person = GetPersonFromSomewhere();
if (person.Address != null)
{
   // Do something with the address field
}

or (more modern idiom)

var person = GetPersonFromSomewhere();
if (person.Address is object)
{
   // Do something with the address field
}

Likewise person.ClearAddress() is just equivalent to person.Address = null;

We should consider removing the presence accessors here.

Note that this becomes trivial to do after #7382 is merged.

[jskeet]

Note that we can also remove the Has/Clear for oneof fields, given that users can use the existing FooCase/ClearFoo pair.
There's less of a compelling case to do so, however: oneof fields can never be explicitly optional, so we don't get into the inconsistency between "explicitly optional in proto3 vs implicitly optional in proto2" situation.

[jskeet]

The case for removing it from oneof fields is still fairly compelling though:

• It reduces the change when updating from proto2 to proto3
• It reduces the number of members to look through in Intellisense
• It avoids the slightly odd effect that Clear only has an effect if the oneof was set to that case already.

[jskeet]

Oh, and I think other languages don't generate Has/Clear for individual oneof fields, either.

Looks like that's not the case. It also looks like this is an expected inconsistency when upgrading from proto2 to proto3 in other languages, so I'll leave it in C# too.