[SQL][minor] remove duplicated cases in `DecimalPrecision`

We already have a rule to do type coercion for fixed decimal and unlimited decimal in `WidenTypes`, so we don't need to handle them in `DecimalPrecision`.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #6698 from cloud-fan/fix and squashes the following commits:

413ad4a [Wenchen Fan] remove duplicated cases

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/HiveTypeCoercion.scala

@@ -445,12 +445,6 @@ trait HiveTypeCoercion {
                                   e2 @ DecimalType.Expression(p2, s2)) if p1 != p2 || s1 != s2 =>
           val resultType = DecimalType(max(p1, p2), max(s1, s2))
           b.makeCopy(Array(Cast(e1, resultType), Cast(e2, resultType)))
-        case b @ BinaryComparison(e1 @ DecimalType.Fixed(_, _), e2)
-          if e2.dataType == DecimalType.Unlimited =>
-          b.makeCopy(Array(Cast(e1, DecimalType.Unlimited), e2))
-        case b @ BinaryComparison(e1, e2 @ DecimalType.Fixed(_, _))
-          if e1.dataType == DecimalType.Unlimited =>
-          b.makeCopy(Array(e1, Cast(e2, DecimalType.Unlimited)))
 
         // Promote integers inside a binary expression with fixed-precision decimals to decimals,
         // and fixed-precision decimals in an expression with floats / doubles to doubles