fix imports in DataFrame.kt

Author: AndreiKingsley

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/DataFrame.kt

@@ -14,7 +14,6 @@ import org.jetbrains.kotlinx.dataframe.annotations.Interpretable
 import org.jetbrains.kotlinx.dataframe.annotations.Refine
 import org.jetbrains.kotlinx.dataframe.annotations.RequiredByIntellijPlugin
 import org.jetbrains.kotlinx.dataframe.api.ColumnsSelectionDsl
-import org.jetbrains.kotlinx.dataframe.api.PivotDocs
 import org.jetbrains.kotlinx.dataframe.api.add
 import org.jetbrains.kotlinx.dataframe.api.cast
 import org.jetbrains.kotlinx.dataframe.api.getRows
@@ -24,7 +23,6 @@ import org.jetbrains.kotlinx.dataframe.api.select
 import org.jetbrains.kotlinx.dataframe.api.toDataFrame
 import org.jetbrains.kotlinx.dataframe.columns.UnresolvedColumnsPolicy
 import org.jetbrains.kotlinx.dataframe.columns.toColumnSet
-import org.jetbrains.kotlinx.dataframe.documentation.DocumentationUrls
 import org.jetbrains.kotlinx.dataframe.impl.DataFrameImpl
 import org.jetbrains.kotlinx.dataframe.impl.DataFrameSize
 import org.jetbrains.kotlinx.dataframe.impl.getColumnsImpl