[MINOR][REPL] Remove dead code of Spark Repl in Scala 2.11

## What changes were proposed in this pull request?

Remove the dead code of Spark Repl in Scala 2.11. We only support Scala 2.12

## How was this patch tested?
N/A

Closes apache#24588 from gatorsmile/removeDeadCode.

Authored-by: gatorsmile <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: gatorsmile