Fixes warnings about self

[kyledecot]

Does any other open PR do the same thing?

No

What issue is this PR fixing?

Explicitly mentioning self warning

How did you test this PR?

Built application and warnings went away

Fixes the following warnings that I saw when building an application that used this package:

13:05:49]: ▸ ⚠️  /Users/kyledecot/code/root-react-native/node_modules/react-native-maps/lib/ios/AirGoogleMaps/AIRGoogleMapMarker.m:226:72: block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended be]
havior [-Wimplicit-retain-self]
[13:05:49]: ▸ if (_iconImageView) [_iconImageView removeFromSuperview];
[13:05:49]: ▸            ^
[13:05:49]: ▸ ⚠️  /Users/kyledecot/code/root-react-native/node_modules/react-native-maps/lib/ios/AirGoogleMaps/AIRGoogleMapMarker.m:226:89: block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended be]
havior [-Wimplicit-retain-self]
[13:05:49]: ▸ if (_iconImageView) [_iconImageView removeFromSuperview];
[13:05:49]: ▸                                                                        ^
[13:05:49]: ▸ ⚠️  /Users/kyledecot/code/root-react-native/node_modules/react-native-maps/lib/ios/AirGoogleMaps/AIRGoogleMapMarker.m:252:68: block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended be]
havior [-Wimplicit-retain-self]
[13:05:49]: ▸ _iconImageView = imageView;
[13:05:49]: ▸

[rborn]

LGTM

@alvelig 🐽

[alvelig]

LGTM