Hidden code for redirecting user

Hi,
I’m having a trouble for redirecting users. Whatever I do -code changes, plugins, snipets-mipts NOTHING works.
Recently I found this:

<p class="submit">
<input id="wp-submit" class="button button-primary button-large" type="submit" value="Log In" name="wp-submit">
<strong><input type="hidden" value="http://www.example.com/wp-admin/" name="redirect_to"></strong>
<input type="hidden" value="1" name="testcookie">
</p>

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This is the submit button on page http://www.domain.com/wp-admin
I don’t know how to delete this code.
Can somebody help me, please?
Thanks in advance!