Fix onFocus called too many times

[daniloercoli]

This PR fixes an issue where multiple instances of AztecWrapper in the same screen can lead to a focus loop. See Focus Loop when deleting content and blocks.

Removing the onFocus bind, and relying only onPress fixes the problem.

Testing steps provided in the other PR here: wordpress-mobile/gutenberg-mobile#314.

[mzorz]

Thank you for finding this fix @daniloercoli !

I've tested it on various emulators and handsets and I can confirm it fixes the focus change infinite loop problem.

I've also been following the code and undesrtand how onPress on the Touchable area calls the onFocus listeners which in turn may be calling (as is the case for the mobile Gutenberg project) onFocus on RCTAztecView again, thus generating the problem.
Given the line onFocus = { () => {} } seems too weird and will surely catch more than one pair of eyes in the future, I've left a suggestion to make a more thorough comment there, explaining the problem, so future developers that find this line understand have more information to understand why this line is important and does what we actually intend to do.

Let's add that comment and then we'll be good to merge 👍