Remove backward compatibility option for nimble renaming

Summary:
Presto has adopted the new name NIMBLE in the code base and
is getting test failures as NIMBLE cannot be found when the flag
VELOX_ENABLE_BACKWARD_COMPATIBILITY is set to be true.

Reviewed By: Yuhta, spershin, helfman

Differential Revision: D56803216

fbshipit-source-id: 10b61423480025156e225aa07694ccd54cb85731

velox/dwio/common/Options.h

@@ -45,11 +45,7 @@ enum class FileFormat {
   TEXT = 5,
   JSON = 6,
   PARQUET = 7,
-#ifdef VELOX_ENABLE_BACKWARD_COMPATIBILITY
-  ALPHA = 8,
-#else
   NIMBLE = 8,
-#endif
   ORC = 9,
 };