[RUN-AS-IF-FOSS] Remove "preloadFixtures" and "loadJSONFixtures" from jest specs

What does this MR do?

• Remove calls to preloadFixtures
• Remove eslint rules for preloadFixtures and loadJSONFixtures under spec/frontend (and ee/spec/frontend).

Why?

Both preloadFixtures and loadJSONFixtures aren't needed anymore in the migrated jest specs.

Does this MR meet the acceptance criteria?

Conformity

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• Tested in all supported browsers
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Security

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