Remove duplicate 'bp_get_activity_show_filters' filter.

'bp_get_activity_show_filters_options' now filters the options used to build
the options markup, while 'bp_get_activity_show_filters' filters the markup.

See #5938.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9133 cdf35c40-ae34-48e0-9cc9-0c9da1808c22