Fix DG diagram bugs

[papataco14]

• Fixed the model manager bug for diagrams.
• Removed unused diagrams.
• Modified ModelClassDiagram to fit current image.

[codecov[bot]]

Codecov Report

Merging #287 (6851fd2) into master (598d230) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #287   +/-   ##
=========================================
  Coverage     74.30%   74.30%           
  Complexity      579      579           
=========================================
  Files            88       88           
  Lines          1903     1903           
  Branches        221      221           
=========================================
  Hits           1414     1414           
  Misses          407      407           
  Partials         82       82           

Files	Coverage Δ
...va/seedu/address/logic/commands/RemarkCommand.java	93.54% <100.00%> (ø)