libutil/ev_zlist: drop unused composite watcher

[garlick]

Just noticed the ev_zlist composite libev watcher no longer has any users, so let's drop it.

[grondo]

Ok, will merge after travis reports.

[coveralls]

Coverage decreased (-0.05%) to 79.031% when pulling 5f5468c on garlick:drop_ev_zlist into ccc0196 on flux-framework:master.

[codecov-io]

Codecov Report

Merging #1493 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1493      +/-   ##
==========================================
- Coverage   78.78%   78.73%   -0.06%     
==========================================
  Files         164      163       -1     
  Lines       30587    30553      -34     
==========================================
- Hits        24098    24055      -43     
- Misses       6489     6498       +9

Impacted Files	Coverage Δ
src/broker/module.c	83.79% <0%> (-1.4%)	⬇️
src/broker/content-cache.c	73.43% <0%> (-1.3%)	⬇️
src/common/libflux/rpc.c	93.38% <0%> (-0.83%)	⬇️
src/common/libkvs/treeobj.c	85.14% <0%> (-0.41%)	⬇️
src/cmd/flux-module.c	85.06% <0%> (-0.31%)	⬇️
src/common/libflux/message.c	81.13% <0%> (-0.12%)	⬇️
src/common/libkvs/kvs_watch.c	89.69% <0%> (+0.42%)	⬆️
src/common/libutil/base64.c	95.77% <0%> (+0.7%)	⬆️
src/broker/modservice.c	81.55% <0%> (+0.97%)	⬆️
src/common/libutil/blobref.c	98.61% <0%> (+1.38%)	⬆️

[garlick]

Thanks.