[2506] removed meaningless comment

this is remaining open point from #2375.  The comment was added at
commit 33a3143, and it's been the same since the introduction.
Aside from the fact that the sentence is incomplete, IMO the comment doesn't
provide any meaningful information; it's obvious that "the source is not
available" if source_ is NULL.  Even if we complete the sentence by
adding, e.g, "(then) throw InvalidOperation", it still doesn't provide
any additional information than the code itself.

So my decision is to to simply clean it up.

src/lib/dns/master_lexer.cc

@@ -167,7 +167,6 @@ MasterLexer::getSourceLine() const {
 
 const MasterLexer::Token&
 MasterLexer::getNextToken(Options options) {
-    // If the source is not available
     if (impl_->source_ == NULL) {
         isc_throw(isc::InvalidOperation, "No source to read tokens from");
     }