Fix rollup bug when init RowCursor in MergeContext (#1510)
Fix rollup bug when init RowCursor in MergeContext (#1510)

When doing rollup, seek_columns equals to the complete set of tablet's columns.
There is no necessity to set it.
Related to commit 36df6ebe4e5f0abd3f07c1e454710590f1de23c7
imay/palo,imay/palo,imay/palo,imay/palo,imay/palo,imay/palo