refactor(legacy): remove unneeded dynamic import var init code

[sapphi-red]

Description

Currently plugin-legacy outputs html like below. (extracted the relevant part)

    <script type="module">var __vite_is_dynamic_import_support=false;</script>
    <script type="module">try{import("_").catch(()=>1);}catch(e){}window.__vite_is_dynamic_import_support=true;</script>
    <script type="module">!function(){if(window.__vite_is_dynamic_import_support)return;console.warn("vite: loading legacy build because dynamic import is unsupported, syntax error above should be ignored");var e=document.getElementById("vite-legacy-polyfill"),n=document.createElement("script");n.src=e.src,n.onload=function(){System.import(document.getElementById('vite-legacy-entry').getAttribute('data-src'))},document.body.appendChild(n)}();</script>

The first script tag is unused.
The reason is that a top level variable inside type="module" is not exposed to global.
It could be fixed by removing type="module" or changing to window.__vite_is_dynamic_import_support = false.
But since it is a falsy value, the declaration is not needed.
So I dropped this script tag.

This will simply the code a bit and also reduce file size of html (and csp header) a bit.

I tested this with https://github.com/yoyo930021/vite-legacy-csp-bug in Chrome 102, Chrome 62, Edge 18, Firefox 66, Safari 10.1.

refs #6535

Additional context

https://discord.com/channels/804011606160703521/804439875226173480/964612065337696307

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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