Fix calculation to apply amount conversion only once for costing rate

[zeel-codder]

Description

This pull request updates the get_employee_costing_rate function in next_pms/project_currency/overrides/timesheet.py to improve its handling of employee costing rates. The changes focus on adding a new parameter to the salary calculation and simplifying the logic by removing currency conversion.

Updates to employee costing rate calculation:

• Added the date parameter (start_date) to the salary calculation function call, ensuring that the calculation is date-specific.
• Removed the logic for converting the salary currency to the project currency, simplifying the function by eliminating the use of get_exchange_rate.

Relevant Technical Choices

Testing Instructions

Additional Information:

Screenshot/Screencast

Checklist

• I have carefully reviewed the code before submitting it for review.
• This code is adequately covered by unit tests to validate its functionality.
• I have conducted thorough testing to ensure it functions as intended.
• A member of the QA team has reviewed and tested this PR (To be checked by QA or code reviewer)