fix: tax rate being overridden in case of 0.00

Tax rate could be different for different expenses.
Therefore, rate is kept as 0.00 and tax amount entered manually. 
But fetching used to override the rate(upon saving) and mess up the amount.

erpnext/hr/doctype/expense_taxes_and_charges/expense_taxes_and_charges.json

@@ -56,8 +56,6 @@
   },
   {
    "columns": 2,
-   "fetch_from": "account_head.tax_rate",
-   "fetch_if_empty": 1,
    "fieldname": "rate",
    "fieldtype": "Float",
    "in_list_view": 1,
@@ -111,4 +109,4 @@
  "sort_field": "modified",
  "sort_order": "ASC",
  "track_changes": 1
-}
+}