Fix highlighting comments in scss files

The sccs syntax file imports the definitions of the sass indented syntax
where comments follow some indentation rules[1].

These indentation rules are not valid in the scss syntax[2], and their
visual rendering is confusing.

So delete the syntax items referring to sass comments files before using
the newly defined ones.

Also derive the visual style directly from Comment instead of
sassComment, just to stress that the comment definitions are distinct
from the sass ones.

[1] http://sass-lang.com/documentation/file.INDENTED_SYNTAX.html#comments
[2] http://sass-lang.com/documentation/file.SASS_REFERENCE.html#comments

Closes #78.

syntax/scss.vim

@@ -10,10 +10,15 @@ endif
 
 runtime! syntax/sass.vim
 
-syn match scssComment "//.*" contains=sassTodo,@Spell
-syn region scssComment start="/\*" end="\*/" contains=sassTodo,@Spell
+syn clear sassComment
+syn clear sassCssComment
+syn clear sassEndOfLineComment
 
-hi def link scssComment sassComment
+syn match scssComment "//.*" contains=sassTodo,@Spell
+syn region scssCssComment start="/\*" end="\*/" contains=sassTodo,@Spell
+
+hi def link scssCssComment scssComment
+hi def link scssComment Comment
 
 let b:current_syntax = "scss"