Remove subgrid fallbacks once it's fully supported in all browsers

[adamkudrna]

Manual fallbacks sort of work but we cannot achieve the same results for all use cases as with the real subgrid.

• Search all occurrences of subgrid. Pay special attention to FormLayout and form fields, they are complex and need to be tested properly.
• Update docs: /components/layout/form-layout#label-width
• Added after #135: fallback now uses display: contents instead of the manually added fallback width.

Support status as of August 2020

• Firefox ✅
• Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=618969
• Safari: https://bugs.webkit.org/show_bug.cgi?id=202115

https://caniuse.com/#feat=css-subgrid