Fix test to validate forms & remove some unused code

.toxic.json

CRM/Contribute/Import/Form/DataSource.php

CRM/Contribute/Import/Parser/Contribution.php

CRM/Export/BAO/Export.php

...phpunit/CRM/Contribute/Import/Parser/ContributionTest.php

tests/phpunit/CRM/Custom/Import/Parser/ApiTest.php

tests/phpunit/CRMTraits/Import/ParserTrait.php