Follow changes in the default runtime files

These removed syntaxes are included in [runtime files update][1].

[1]: https://github.com/vim/vim/commit/06fe74aef72606ac34c9f494186e52614b8fb59a#diff-613ff34c5d60e41a3fc93fcb58277143

after/syntax/css/variables-1.vim

@@ -1,2 +0,0 @@
-syn match cssFontProp contained "\<--[A-Za-z_][A-Za-z0-9_-]\+\>"
-syn region cssFunction contained matchgroup=cssFunctionName start="\<var\s*(" end=")" oneline keepend