Don't specialize similar for Hankel

[jishnub]

Since the output of similar should be mutable, and Hankel doesn't support setindex!, we shouldn't specialize similar(::Hankel) to return a Hankel. This PR removes the specialization.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3d47002) 95.25% compared to head (ae26b28) 95.59%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage   95.25%   95.59%   +0.33%     
==========================================
  Files           9        9              
  Lines         948      953       +5     
==========================================
+ Hits          903      911       +8     
+ Misses         45       42       -3     

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[github-actions]

Pull Request Test Coverage Report for Build 7443272770

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.593%

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Totals
Change from base Build 7443258038:	0.0%
Covered Lines:	911
Relevant Lines:	953

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💛 - Coveralls