option to remove fallback (handler)

[emphasize]

two things in question:

• register with core fallback service only if handler present - ie. register_fallback called upon the skill
• remove fallback (handler) in full (and consequentially deregister)

[codecov]

Codecov Report

Attention: 40 lines in your changes are missing coverage. Please review.

Comparison is base (4f65ac5) 60.23% compared to head (924ebe7) 59.66%.
Report is 17 commits behind head on dev.

Files	Patch %	Lines
ovos_workshop/skills/ovos.py	24.48%	37 Missing ⚠️
ovos_workshop/skills/fallback.py	85.71%	2 Missing ⚠️
ovos_workshop/version.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #150      +/-   ##
==========================================
- Coverage   60.23%   59.66%   -0.57%     
==========================================
  Files          34       34              
  Lines        3347     3384      +37     
==========================================
+ Hits         2016     2019       +3     
- Misses       1331     1365      +34     

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