Modify DeleteCommand, DeleteParser to UntagCommand, UntagParser

[li-s]

Resolve #53

[codecov-io]

Codecov Report

Merging #84 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master      #84   +/-   ##
=========================================
  Coverage     57.50%   57.50%           
  Complexity      329      329           
=========================================
  Files            70       70           
  Lines          1193     1193           
  Branches         92       92           
=========================================
  Hits            686      686           
  Misses          480      480           
  Partials         27       27           

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...ava/seedu/address/logic/commands/UntagCommand.java	100.00% <100.00%> (ø)	7.00 <1.00> (?)
.../seedu/address/logic/parser/AddressBookParser.java	100.00% <100.00%> (ø)	11.00 <0.00> (ø)
...seedu/address/logic/parser/UntagCommandParser.java	100.00% <100.00%> (ø)	2.00 <1.00> (?)

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[luo-git]

LGTM