refactor(Salary Structure Assignment): opening balances setup warning + section visibility (backport #2309)

[mergify]

Simplified overengineered setup for setting opening balances in salary structure assignments:

• Remove a checkbox from Payroll Settings. This setting just toggles the visibility of the opening earnings till date & tax deducted till date, but if it's disabled & amounts are set, these amounts are still considered in the payroll. Seems like an overkill for just controlling visibility
  

• Relabelled opening balances section
  

• Simplified logic for toggling & warning about opening balances. Too much back and forth between functions, buggy and typos in functions

Closes #1853

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This is an automatic backport of pull request #2309 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 165b361 has failed:

On branch mergify/bp/version-15-hotfix/pr-2309
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	both modified:   hrms/payroll/doctype/payroll_settings/payroll_settings.json

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Cherry-pick of fc6a21a has failed:

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Unmerged paths:
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	both modified:   hrms/payroll/doctype/salary_structure_assignment/salary_structure_assignment.json

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Cherry-pick of 627e391 has failed:

On branch mergify/bp/version-15-hotfix/pr-2309
Your branch is ahead of 'origin/version-15-hotfix' by 2 commits.
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Changes to be committed:
	modified:   hrms/payroll/doctype/salary_structure_assignment/salary_structure_assignment.py

Unmerged paths:
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	both modified:   hrms/payroll/doctype/salary_structure_assignment/salary_structure_assignment.js

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