fix: removed serial no validation for sales invoice

[rohitwaghchaure]

As we allowing to inward same serial no multiple times in the PR (#25471), removing the serial no validation in the sales invoice due to which user not able to save the invoice

  File "/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/sales_invoice/sales_invoice.py", line 124, in validate
    self.validate_serial_numbers()
  File "/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/sales_invoice/sales_invoice.py", line 1093, in validate_serial_numbers
    self.validate_serial_against_sales_invoice()
  File "/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/sales_invoice/sales_invoice.py", line 1142, in validate_serial_against_sales_invoice
    .format(serial_no, serial_no_details.sales_invoice))
  File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 411, in throw
    msgprint(msg, raise_exception=exc, title=title, indicator='red', is_minimizable=is_minimizable, wide=wide, as_list=as_list)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 390, in msgprint
    _raise_exception()
  File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 344, in _raise_exception
    raise raise_exception(msg)
frappe.exceptions.ValidationError: Serial Number: 465793 is already referenced in Sales Invoice: ACC-SINV-2021-06259

[coveralls]

Pull Request Test Coverage Report for Build 872269468

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 137 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.005%) to 61.211%

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Files with Coverage Reduction	New Missed Lines	%
/home/runner/frappe-bench/apps/erpnext/erpnext/portal/product_configurator/item_variants_cache.py	1	30.0%
/home/runner/frappe-bench/apps/erpnext/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py	13	97.45%
/home/runner/frappe-bench/apps/erpnext/erpnext/accounts/doctype/sales_invoice/sales_invoice.py	123	77.61%

Totals
Change from base Build 871812420:	-0.005%
Covered Lines:	54907
Relevant Lines:	89701

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💛 - Coveralls