Error number 3360 is reused for two errors

[auduchinok]

It seems the same number has somehow ended up being reused for two unrelated errors:

• parsEqualsMissingInTypeDefinition
• typrelInterfaceWithConcreteAndVariable

[psfinaki]

Should be fixed by #15585?

[auduchinok]

Yes, it's no longer used in the grammar there. We should probably also remove it from the FSComp file?

[psfinaki]

I think so. Wonder if we have more duplicates like this.