Draft: Remove useless patch code of po_to_json

What does this MR do and why?

Related issue: #2948 (closed)

What

Delete useless code below because they are not used anywhere anymore.

• Gitlab::Patch::GettextI18nRailsJsTask
• Gitlab::Patch::PoToJson

Why

These "Patch" codes were previously used in the rake gettext:po_to_json command to generate the required front-end i18n file, such as app/assets/javascripts/locale/en/app.js.

The po_to_json command is now deprecated and replaced by bin/rake gettext:compile (See Upstream MR).

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

Verify these commands work correctly: bin/rake jh:gettext:regenerate and bin/rake gettext:compile

1. bin/rake gettext:compile

   # Delete `app.js` file
   $ rm app/assets/javascripts/locale/en/app.js
   
   # Execute compile command
   $ bin/rake gettext:compile
   
   # Expect `app.js` to be regenerated
   $ ls app/assets/javascripts/locale/en/app.js

2. bin/rake jh:gettext:regenerate

   $ old_i18n_string="JH|VulnerabilityExports|Tool"
   $ new_i18n_string="JH|VulnerabilityExports|Tooooool"
   $ filename="jh/app/services/jh/vulnerability_exports/exporters/csv_service.rb"
   
   $ sed -i '' "s/$old_i18n_string/$new_i18n_string/g" $filename
   
   # Expect the `gitlab.po` file to be updated
   $ bin/rake jh:gettext:regenerate

MR acceptance checklist

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