/*已安裝外掛 > 安裝外掛按鈕*/
div[class="wrap"] a[href$="plugin-install.php"]
{
    display: none;
}

/*==========adminmenu > sub level (後台工具列子目錄)==========*/
/*安裝外掛*/
li[id="menu-plugins"] > ul[class^="wp-submenu"] > li:has(a[href="plugin-install.php"])
{
    display: none;
}