core/exec-invoke: remove redundant accmode validation on stderr

Follow-up for 171ceb4a00

In setup_output() we assume stdout has been set up properly
before stderr, hence the stdout we're inheriting from must
be writable (or more precisely, would have been adjusted to be).
Hence no need to duplicate it.

src/core/exec-invoke.c

@@ -546,17 +546,8 @@ static int setup_output(
                         return fileno;
 
                 /* Duplicate from stdout if possible */
-                if (can_inherit_stderr_from_stdout(context)) {
-                        r = fd_is_writable(STDOUT_FILENO);
-                        if (r <= 0) {
-                                if (r < 0)
-                                        log_warning_errno(r, "Failed to check if inherited stdout is writable for stderr, falling back to /dev/null.");
-                                else
-                                        log_warning("Inherited stdout is not writable for stderr, falling back to /dev/null.");
-                                return open_null_as(O_WRONLY, fileno);
-                        }
+                if (can_inherit_stderr_from_stdout(context))
                         return RET_NERRNO(dup2(STDOUT_FILENO, fileno));
-                }
 
                 o = context->std_error;