Revert "会員登録済みのメールアドレスを判別不可にする"

[chihiro-adachi]

Reverts #5391

Revertの経緯は以下に記載しています。
#5279 (comment)

[codecov-commenter]

Codecov Report

Merging #5460 (650be08) into 4.2 (9bafbe4) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                4.2    #5460      +/-   ##
============================================
+ Coverage     78.74%   78.78%   +0.03%     
+ Complexity     6251     6246       -5     
============================================
  Files           468      468              
  Lines         21007    20979      -28     
============================================
- Hits          16543    16528      -15     
+ Misses         4464     4451      -13     

Flag	Coverage Δ
E2E	64.39% <92.00%> (+0.06%)	⬆️
Unit	77.04% <92.00%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
src/Eccube/Controller/EntryController.php	93.75% <100.00%> (+5.35%)	⬆️
src/Eccube/Entity/Customer.php	91.95% <100.00%> (+0.08%)	⬆️
src/Eccube/Form/Type/Admin/CustomerType.php	93.47% <100.00%> (+0.14%)	⬆️
src/Eccube/Service/MailService.php	90.34% <100.00%> (ø)
src/Eccube/Repository/CustomerRepository.php	96.87% <0.00%> (+1.87%)	⬆️
...be/Service/PurchaseFlow/Processor/TaxProcessor.php	78.94% <0.00%> (+3.50%)	⬆️

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