fix(ios): memory leak cause by style prop changes

[Gnative]

Sorry @mfazekas I caused a memory leak with the previous update for styleURL when mounting/unmounting map instances. Hopefully this fix is ok.

• I've read CONTRIBUTING.md
• I updated the doc/other generated code with running yarn generate in the root folder
• I have tested the new feature on /example app.
  • In V11 mode/ios
  • In New Architecture mode/ios
  • In V11 mode/android
  • In New Architecture mode/android
• I added/updated a sample - if a new feature was implemented (/example)