Drop invalid blockComment setting (#123)

There isn't a block comment in BBC BASIC, but we were setting
`REM` ... `:` as one.  The result was that the editor's "Toggle block
comment" command would insert a REM before the selection and ":" after
which doesn't actually comment out the selected block.

If we don't set this, the block comment commands just don't do
anything, which seems more helpful than doing something wrong.

src/bbcbasic.js

@@ -220,7 +220,6 @@ export function registerBbcBasicLanguage() {
 
     languages.setLanguageConfiguration("BBCBASIC", {
         comments: {
-            blockComment: ["REM", ":"],
             lineComment: "REM",
         },
         brackets: [