docs: delete the q8 ratio cells instead of footnoting them

The data contract refuses a unit=x cell with comparison_allowed=false --
"a ratio over figures never established as comparable" -- and it is right
to. Marking the q8 polars-vs-kuzu ratio non-quotable left the number
sitting in the published data for someone to quote. Deleted instead. The
raw per-engine q8 figures stay, caveated and non-comparable.

Also withdraws the matching prose: gfql/performance.rst still asserted
"GFQL wins q8", which is the exact claim this PR exists to retract.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YYZRXegrALuXd3NHH5evqx

Author: lmeyerov