[K2] Revert changes done in KT-61786

Changes in this ticket remove type enhancement in java fields
for K2. But after KT-61920 fix, we can actually support it.

compiler/testData/ir/irText/declarations/kt29833.fir.kt.txt

@@ -7,11 +7,11 @@ object Definitions {
 
   }
 
-  const val KT_CONSTANT: @FlexibleNullability String?
+  const val KT_CONSTANT: String
     field = "constant"
     get
 
-  val ktValue: @FlexibleNullability String?
+  val ktValue: String
     field = #CONSTANT
     get