Honor deprecation notices by removing them and accompanied code

[caronc]

Description:

Related issue (if applicable): n/a

Just some code clean up; there have been deprecation warnings that date back more than a year now:

• Twitter: tweet:// is not an accepted schema to initialize it with.
  • Deprecation warning since Jun 29th, 2019 (14 months)
• Discord: thumbnail= was switched to image= for consistency with other plugins
  • Deprecation warning since April 5th, 2019 (17 months)
• Matrix: There were 2 deprecations cleaned up here:
  1. Determining the type of webhook mode used is identified by mode= not webhook=; this was done for consistency with other plugins.
     - Deprecation warning since April 5th, 2019 (17 months)
  2. thumbnail= was switched to image= for consistency with other plugins
     - Deprecation warning since April 5th, 2019 (17 months)
• Pushjet: secret_key@hostame is no longer accepted. The syntax is hostname/secret_key. This was to remove conflicting cases where user:pass@hostname/secret_key was used.
  • Deprecation warning since Sept 7th, 2019 (12 months)
• Ryver: Determining the type of webhook mode used is identified by mode= not webhook=; this was done for consistency with other plugins.
  - Deprecation warning since April 5th, 2019 (17 months)

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

[codecov-commenter]

Codecov Report

Merging #283 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #283   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           83        83           
  Lines         9942      9927   -15     
  Branches      1617      1611    -6     
=========================================
- Hits          9942      9927   -15     

Impacted Files	Coverage Δ
apprise/plugins/NotifyPushjet.py	100.00% <ø> (ø)
apprise/plugins/NotifyDiscord.py	100.00% <100.00%> (ø)
apprise/plugins/NotifyMatrix.py	100.00% <100.00%> (ø)
apprise/plugins/NotifyRyver.py	100.00% <100.00%> (ø)
apprise/plugins/NotifyTwitter.py	100.00% <100.00%> (ø)

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