mod_slotmem_shm: follow up tp r1822509.

Please buildbot (and incidentally users of older APR) by using
apr_shm_remove() instead of the new(er) apr_shm_delete().



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1822511 13f79535-47bb-0310-9956-ffa450edef68