When populating group_creation_steps, use cached disable-avatar setting

Calling bp_get_option() to retrieve a value that has already been cached in
BP's main bootstrap can cause unreasonable overhead on multisite setups.

Fixes #4914

Props wpdennis

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6936 cdf35c40-ae34-48e0-9cc9-0c9da1808c22