AK: Remove StringBuilder::build() in favor of to_deprecated_string()

Having an alias function that only wraps another one is silly, and
keeping the more obvious name should flush out more uses of deprecated
strings.
No behavior change.

Tests/LibC/TestSnprintf.cpp

@@ -43,7 +43,7 @@ static DeprecatedString show(ByteBuffer const& buf)
             builder.append('_');
     }
     builder.append(')');
-    return builder.build();
+    return builder.to_deprecated_string();
 }
 
 template<typename TArg>