refactor: remove storage type from SKU lookup table

[mboersma]

Reason for Change:

Since a VM's storage type is calculable from its name, there's no need to maintain a static lookup table. This is a minor simplification to ensure there's only one code path for finding storage type.

(This is also the only separable change that came out of my work on #4277 so far.)

Issue Fixed:

Refs #4277

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[codecov]

Codecov Report

Merging #4316 (1a99826) into master (f1943db) will increase coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #4316      +/-   ##
==========================================
+ Coverage   72.04%   72.05%   +0.01%     
==========================================
  Files         141      141              
  Lines       21636    21632       -4     
==========================================
+ Hits        15587    15588       +1     
+ Misses       5099     5093       -6     
- Partials      950      951       +1     

Impacted Files	Coverage Δ
pkg/api/common/helper.go	71.09% <ø> (ø)
cmd/get_skus.go	50.63% <50.00%> (+4.12%)	⬆️
pkg/helpers/azure_skus.go	91.66% <50.00%> (-8.34%)	⬇️

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[jackfrancis]

/lgtm

[acs-bot]

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