Unable to delete Price Field from Price Set used for Contributions
Price sets relating to any contribution record(apart from contribution pages) also need to be deleted before removing any fields from the priceset.

Since Contribution need this field information to show them on viewing those contribution record, it doesn't let us to delete the fields or pricesets related to them.

To know what contribution or membership record are related to pricesets - execute this query - 

SELECT DISTINCT cli.entity_table, cli.entity_id
FROM      civicrm_line_item cli
LEFT JOIN civicrm_price_field cpf ON cli.price_field_id = cpf.id
WHERE     cpf.price_set_id = &lt;your_price_set_id&gt;