Cleanups in Finance module

[rathorevaibhav]

Describe the task

1. Cleanup Salaries module (if not being used anywhere)
2. Cleanup Payments module
3. Cleanup Legal Documents module
4. Cleanup Expenses module

Expected behavior

1. All the code related to these modules should be removed
2. Database migrations must also be cleaned, so that there is no impact to the existing migrations. A fresh migration should not give any error.
3. Common files like CSS/JS or other PHP files that are outside of modules and have dependencies mentioned on these modules must also not give any error
4. There should not be any error during testing or using the finance module or the rest of the application