sqlite: revert context-based cancellation hooks from #85

[creachadair]

The interaction between cleanup and reuse of the connection makes this feature hard to use as-written. While #87 proposes a couple ways to fix the issue, let's revert the partial state of things; we can fold these changes in with the fix once we have consensus.

• Revert "sqlite: cancel completed queries after cleanup, not before (#86)" This reverts commit 78a5f32445416db077d122abf09e8aa78202246e.

• Revert "sqlite: add an optional explicit interrupt on context termination (#85)" This reverts commit 607d0c19ac5c651c2a4f9f9836552ef9be08221d.

[creachadair]

@crawshaw this one should not require detailed review 🙂