Fix: don't remove lock on dir when deleting a child node

[tbsmark86]

Current behavior:

1. lock /somedir/
2. delete /somedir/somefile.txt (with token)
3. put /somedir/newfile.txt (with token)
   => ERROR precondition failed

Expected/Fixed behavior:
Step 3) should be executed under the LOCK.

[codecov]

Codecov Report

Merging #1335 (76d6592) into master (ba97a9f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1335   +/-   ##
=========================================
  Coverage     97.35%   97.35%           
- Complexity     2809     2810    +1     
=========================================
  Files           174      174           
  Lines          7976     7978    +2     
=========================================
+ Hits           7765     7767    +2     
  Misses          211      211           

Impacted Files	Coverage Δ
lib/DAV/Locks/Plugin.php	98.34% <100.00%> (+0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ba97a9f...76d6592. Read the comment docs.

[phil-davis]

Assigned myself so that I find this easily to followup.

[phil-davis]

LGTM.
Note: ownCloud is using its own implementation of locking and so I could not reproduce this on ownCloud - I don't think it ever executes this code.