[fix] ReaderGesture: don't crash without custom gestures (#4645)
[fix] ReaderGesture: don't crash without custom gestures (#4645)

Silly oversight in https://github.com/koreader/koreader/pull/4644.

Also remove unused util because apparently CircleCI didn't run on the other PR when it should've.
poire-z/koreader,Markismus/koreader,houqp/koreader,poire-z/koreader,NiLuJe/koreader,Frenzie/koreader,pazos/koreader,Frenzie/koreader,koreader/koreader,mwoz123/koreader,Hzj-jie/koreader,mihailim/koreader,koreader/koreader,NiLuJe/koreader