Remove WordPress registration link from login forms through custom CSS.

style.css

/** For removing "|" symbol, that fall between 2 links. */
.login-action-login #nav, .login-action-lostpassword #nav {
    font-size: 0;
}
/** For removing registration link on login form. */
.login-action-login #nav a:first-child{
    display: none;
}
/** For removing registration link on lost password form. */
.login-action-lostpassword #nav a:nth-child(2){
    display: none;
}