Revert "Fixed a missed compilation error in TestValidators."

This reverts commit b3749a0931e4e2c5447d9ce4cdfe43245300ef61.
Revert "TAJO-1114: Improve ConfVars (SessionVar) to take a validator interface to check its input. (fixed compilation error)"

This reverts commit 7a65763554e556498b6d52dd2685636c3b6584b3.
Revert "TAJO-1114: Improve ConfVars (SessionVar) to take a validator interface to check its input. (Jihun Kang via hyunsik)"

This reverts commit ee89c65b44142ca640020339e7ab5a608c1a8bf9.