Fix libc++ pretty printer test for Python 3 after D67238 (take 2)

In both Python 2 and Python 3, gdb.Value.string returns a 'str'. We just
need to delete a `encode("utf-8")` which would return a 'bytes' in
Python 3.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373570 91177308-0d34-0410-b5e6-96231b3b80d8

test/pretty_printers/gdb_pretty_printer_test.py

@@ -46,12 +46,10 @@ class CheckResult(gdb.Command):
             gdb.newest_frame().select()
 
             expectation_val = compare_frame.read_var("expectation")
+            check_literal = expectation_val.string(encoding="utf-8")
             if "PrettyPrintToRegex" in compare_frame.name():
-                check_literal = expectation_val.string()
                 test_fails = not re.match(check_literal, value)
             else:
-                check_literal_string = expectation_val.string(encoding="utf-8")
-                check_literal = str(check_literal_string.encode("utf-8"))
                 test_fails = value != check_literal
 
             if test_fails: