feat: Remove deprecated MQTTSend pipeline function

[lenny-goodell]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Deprecated MQTTSend pipeline function exists in code

Issue Number: #547

What is the new behavior?

Deprecated MQTTSend pipeline function code has been removed

Does this PR introduce a breaking change?

• Yes
• No

Are there any new imports or modules? If so, what are they used for and why?

no

Are there any specific instructions or things that should be known prior to reviewing?

Other information

[codecov-io]

Codecov Report

Merging #592 (6f61144) into master (d5e638f) will increase coverage by 1.98%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #592      +/-   ##
==========================================
+ Coverage   60.50%   62.49%   +1.98%     
==========================================
  Files          41       40       -1     
  Lines        2175     2061     -114     
==========================================
- Hits         1316     1288      -28     
+ Misses        772      692      -80     
+ Partials       87       81       -6     

Impacted Files	Coverage Δ
appsdk/configurable.go	54.41% <ø> (+0.37%)	⬆️
appsdk/sdk.go	34.80% <ø> (-0.57%)	⬇️

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