Python: Fix slow use of regexCapture in Builtin::strValue

This is only _really_ expensive when there are a _lot_ of strings in
the database, but for this case, where we're always extracting the
same substring of the string, it's easier -- and faster -- to just
make a substring operation directly.

python/ql/src/semmle/python/types/Builtins.qll

@@ -104,7 +104,8 @@ class Builtin extends @py_cobject {
     ) and
     exists(string quoted_string |
       quoted_string = this.getName() and
-      result = quoted_string.regexpCapture("[bu]'([\\s\\S]*)'", 1)
+      // Remove prefix ("b" or "u") and leading and trailing quotes (both "'").
+      result = quoted_string.substring(2, quoted_string.length() - 1)
     )
   }
 }