docs: delete unused/duplicate css files

- `sh.css` already exists in `api_assets`
 - `sh_vim-dark.css` is unused, but used in the repo `node-website`
        now

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Signed-off-by: Julien Gilli <julien.gilli@joyent.com>

doc/sh.css

@@ -1,24 +0,0 @@
-.sh_sourceCode {
-  font-weight: normal;
-  font-style: normal;
-}
-
-.sh_sourceCode .sh_symbol,
-.sh_sourceCode .sh_cbracket {
-  color: #333;
-}
-
-.sh_sourceCode .sh_keyword {
-  color: #c96;
-}
-
-.sh_sourceCode .sh_string,
-.sh_sourceCode .sh_regexp,
-.sh_sourceCode .sh_number,
-.sh_sourceCode .sh_specialchar {
-  color: #690;
-}
-
-.sh_sourceCode .sh_comment {
-  color: #666;
-}

doc/sh_vim-dark.css

@@ -1,33 +0,0 @@
-.sh_sourceCode {
-  font-weight: normal;
-  font-style: normal;
-}
-
-.sh_sourceCode .sh_symbol ,
-.sh_sourceCode .sh_cbracket {
-  color: #fff;
-}
-
-.sh_sourceCode .sh_keyword {
-  font-style: italic;
-}
-
-.sh_sourceCode .sh_string,
-.sh_sourceCode .sh_regexp,
-.sh_sourceCode .sh_number,
-.sh_sourceCode .sh_specialchar {
-  color: #a0c874;
-}
-
-.sh_sourceCode .sh_comment {
-  color: #777;
-}
-
-.alt .sh_sourceCode .sh_keyword {
-  font-style: normal;
-  color: #996633;
-}
-
-.alt .sh_sourceCode .sh_symbol , .alt .sh_sourceCode .sh_cbracket {
-  color: #46483e;
-}