567: Tables no longer remove null rows/columns from their interior

It used to be that if removing a component created a row and/or column
that was all nulls, the all-null row/column would be removed, potentially
moving components that were beyond the row/column left/up. The resulting
mutation to table size meant that components were no longer at the
[row, column] position the user initially specified.

This change modifies Table to remove only all-null rows/columns that have a
higher row/column index than the last non-null element (essentially, null
rows and columns will be trimmed off the outermost edge of the table).

Close #567.
Remove the null-clearing rows and associated tests.

Null rows/columns at the edge of the table are no longer trimmed.
That logic was only really relevant if a component was added at a
far-away [row, column] position and then removed. Dan says I can gloat
if the removal of this logic leads to trouble in the future. (As fun
as gloating may be, I hope I won't have to.)

Bug-specific unit tests relating to null row/columns have been removed.
Merge pull request #581 from palantir/567-table-resizing

567: Tables no longer remove null rows/columns from their interior