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
(https://github.com/actions/create-github-app-token/pull/70/commits/f28f8958a77a5e1e1f036ad308ffa00c25948a97
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>