chore(css): Remove non-existent 'CSS Flexible Lengths' group (#913)

chore(css): Remove not existed CSS Flexible Lengths group

Co-authored-by: Brian Smith <[email protected]>

Author: skyclouds2001

css/definitions.json

@@ -21,7 +21,6 @@
       "CSS Device Adaptation",
       "CSS Display",
       "CSS Flexible Box Layout",
-      "CSS Flexible Lengths",
       "CSS Fonts",
       "CSS Fragmentation",
       "CSS Frequencies",

css/units.json

@@ -65,7 +65,6 @@
   "fr": {
     "groups": [
       "CSS Units",
-      "CSS Flexible Lengths",
       "CSS Grid Layout"
     ],
     "status": "standard"