fix: Remove manage my subscriptions CTAs

[julianajlk]

REV-4003.
Below visual changes are only applied for learners that had a subscription.

Before:

After:

This PR:

• Remove the "Manage my subscriptions" button from the Order History page (aka Orders and Subscriptions Page)
• Removes any fetch calls to Subscriptions service to generate the Customer Portal URL
• Removes the ManageSubscriptionsPage that is used by Program Details Page in edx-platform to generate the URL.

Please note:

• This PR will only be merged/deployed after the edx-platform PR to remove the CTA from the Program Details Page.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.37%. Comparing base (f425c4b) to head (2c5b397).
Report is 73 commits behind head on master.

Files	Patch %	Lines
src/subscriptions/service.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master       #4       +/-   ##
===========================================
+ Coverage   54.05%   67.37%   +13.32%     
===========================================
  Files          15       31       +16     
  Lines         148      282      +134     
  Branches       25       64       +39     
===========================================
+ Hits           80      190      +110     
- Misses         65       87       +22     
- Partials        3        5        +2     

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