remove strategy: "quick" from ally.query.focusable et al

[rodneyrehm]

As of ally.js 1.1.0 the "quick" strategy (using document.querySelectorAll()) still is the default strategy, even though its accuracy is well below the "strict" strategy (using TreeWalker). The reason for keeping qSA approach was selector performance, but it looks as if it isn't all that much faster.

I'd like to remove this option to simplify the docs and the code base:

• remove query/focusable.quick.js
• remove selector/focusable.js
• replace test/unit/query.focusable.js with test/unit/query.focusable.strict.test.js
• remove from tests/focusable/*
• remove from build/data-tables/*

Any objections?