[objc] Removes `readwrite` in objc properties since it is the default (#133)
[objc] Removes `readwrite` in objc properties since it is the default (#133)

Adding `readwrite` to objective-c properties adds no value since it
is the default setting.

Removal was suggested in https://github.com/mono/Embeddinator-4000/pull/129#issuecomment-294491293
jonathanpeppers/Embeddinator-4000,mono/Embeddinator-4000,mono/Embeddinator-4000,mono/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,mono/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,mono/Embeddinator-4000,mono/Embeddinator-4000,mono/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,jonathanpeppers/Embeddinator-4000