fix(dao-registry): extension removal

[fforbeck]

Proposed Changes

• If an extension is removed from the registry, it does not need to delete it from the inverseExtensions mapping. Otherwise the register won't know that the extension was already added once. The idea is that extensions can't be re-added.
• Improved the tests for DaoRegistry.sol

[codecov[bot]]

Codecov Report

Merging #525 (e592453) into master (340856f) will increase coverage by 0.26%. The diff coverage is n/a.

:exclamation: Current head e592453 differs from pull request most recent head 42f4998. Consider uploading reports for the commit 42f4998 to get more accurate results

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
+ Coverage   84.38%   84.64%   +0.26%     
==========================================
  Files          52       52              
  Lines        1857     1856       -1     
  Branches      433      433              
==========================================
+ Hits         1567     1571       +4     
+ Misses        290      285       -5     

Impacted Files	Coverage Δ
contracts/core/DaoRegistry.sol	89.08% <ø> (+2.22%)	:arrow_up:
contracts/guards/MemberGuard.sol	100.00% <0.00%> (+7.14%)	:arrow_up:

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