Revert "stbt batch report: Set table-layout: fixed"

This reverts commit d170eb32fc112b0bf8e23a1bc0734cefcaf38d12.

"table-layout:fixed" isn't necessary if we use "display:none" instead of
"width:0" (for example in columns.yaml) to hide a column. "display:none"
works better on Firefox.

Not using "table-layout:fixed" also works better when columns have a
non-breakable word longer than the specified width (for example:
"test_that_action_panel_title_matches_guide_title").

A while ago I had run some tests to see if "table-layout:fixed"
performed better with very large amounts of data, but on a report of
several thousand rows (that took 6s to render) it had no discernible
effect.