vinyl: factor out vy_range_delete_mem
vinyl: factor out vy_range_delete_mem

This piece of code is already used in two places - on dump completion
and on range deletion. Moreover, in order to not use it for the active
mem deletion, we freeze it in vy_range_delete_mem(), which looks
confusing. So let's factor it out into a separate function.

mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,mejedi/tarantool,rtsisyk/tarantool,mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool