[SPARK-27512][SQL] Avoid to replace ',' in CSV's decimal type inference for backward compatibility
## What changes were proposed in this pull request?
The code below currently infers as decimal but previously it was inferred as string.
**In branch-2.4**, type inference path for decimal and parsing data are different.
2a8343121e/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/UnivocityParser.scala (L153)
c284c4e1f6/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVInferSchema.scala (L125)
So the code below:
```scala
scala> spark.read.option("delimiter", "|").option("inferSchema", "true").csv(Seq("1,2").toDS).printSchema()
```
produced string as its type.
```
root
|-- _c0: string (nullable = true)
```
**In the current master**, it now infers decimal as below:
```
root
|-- _c0: decimal(2,0) (nullable = true)
```
It happened after https://github.com/apache/spark/pull/22979 because, now after this PR, we only have one way to parse decimal:
7a83d71403/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/ExprUtils.scala (L92)
**After the fix:**
```
root
|-- _c0: string (nullable = true)
```
This PR proposes to restore the previous behaviour back in `CSVInferSchema`.
## How was this patch tested?
Manually tested and unit tests were added.
Closes #24437 from HyukjinKwon/SPARK-27512.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
a30983db57