refactor(accounts): AccountFollowモデルでフォロー解除する

[laminne]

close #1838

What does this PR do?

• Repositoryにあったフォロー解除の処理をドメインモデルに移動させました
  • メソッドの挙動は refactor: 削除イベントを発行するようにした #1840 と同じ

Additional information

[codecov]

Codecov Report

❌ Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.69%. Comparing base (9fce9e3) to head (0d175d4).

Files with missing lines	Patch %	Lines
pkg/accounts/adaptor/repository/prisma/prisma.ts	0.00%	3 Missing ⚠️
pkg/accounts/service/unfollow.ts	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1841      +/-   ##
==========================================
+ Coverage   47.65%   47.69%   +0.04%     
==========================================
  Files         171      171              
  Lines        5901     5910       +9     
  Branches      891      893       +2     
==========================================
+ Hits         2812     2819       +7     
- Misses       3089     3091       +2     

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