docs: delete unused badge (#222)

It seems that there is an unused badge in this repository. This badge
was introduced in
https://github.com/actions/create-github-app-token/pull/70, but after
some trial and error, it was removed from the README
(f28f8958a7
in the PR). However, the badge itself was not deleted.

Therefore, this badge appears to be unnecessary. This patch removes it.

badges/coverage.svg

@@ -1,25 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="106"
-    height="20" role="img" aria-label="Coverage: 100%">
-    <title>Coverage: 100%</title>
-    <linearGradient id="s" x2="0" y2="100%">
-        <stop offset="0" stop-color="#bbb" stop-opacity=".1" />
-        <stop offset="1" stop-opacity=".1" />
-    </linearGradient>
-    <clipPath id="r">
-        <rect width="106" height="20" rx="3" fill="#fff" />
-    </clipPath>
-    <g clip-path="url(#r)">
-        <rect width="63" height="20" fill="#555" />
-        <rect x="63" width="43" height="20" fill="#4c1" />
-        <rect width="106" height="20" fill="url(#s)" />
-    </g>
-    <g fill="#fff" text-anchor="middle" font-family="Verdana,Geneva,DejaVu Sans,sans-serif"
-        text-rendering="geometricPrecision" font-size="110">
-        <text aria-hidden="true" x="325" y="150" fill="#010101" fill-opacity=".3"
-            transform="scale(.1)" textLength="530">Coverage</text>
-        <text x="325" y="140" transform="scale(.1)" fill="#fff" textLength="530">Coverage</text>
-        <text aria-hidden="true" x="835" y="150" fill="#010101" fill-opacity=".3"
-            transform="scale(.1)" textLength="330">100%</text>
-        <text x="835" y="140" transform="scale(.1)" fill="#fff" textLength="330">100%</text>
-    </g>
-</svg>