Remove the VSFTPD exploiter

[shreyamalviya]

What does this PR do?

Fixes #1533

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by doing a quick run and making sure nothing breaks in the UI (config and report).
  Tested by building the documentation.

• If applicable, add screenshots or log transcripts of the feature working

[codecov]

Codecov Report

Merging #1565 (779ae79) into develop (8ee918b) will increase coverage by 0.31%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1565      +/-   ##
===========================================
+ Coverage    43.63%   43.95%   +0.31%     
===========================================
  Files          478      477       -1     
  Lines        14265    14159     -106     
===========================================
- Hits          6225     6224       -1     
+ Misses        8040     7935     -105     

Impacted Files	Coverage Δ
...y/monkey_island/cc/services/config_schema/basic.py	100.00% <ø> (ø)
...ces/config_schema/definitions/exploiter_classes.py	100.00% <ø> (ø)
...ng/exploit_processing/exploiter_descriptor_enum.py	100.00% <ø> (ø)

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[ilija-lazoroski]

Looks good to me.