etc/completions: remove flux kvs dropcache

Problem: The flux kvs dropcache command has been removed, but
bash completions still exist for it.

Remove bash completions for flux kvs dropcache.

Author: chu11

etc/completions/flux.pre

@@ -1027,7 +1027,6 @@ _flux_kvs()
         mkdir
         copy
         move
-        dropcache
         version
         wait
         getroot
@@ -1106,9 +1105,6 @@ _flux_kvs()
     move_OPTS="\
         ${copy_OPTS}
     "
-    dropcache_OPTS="\
-        -a --all
-    "
     version_OPTS="\
         -N --namespace= \
     "