Remove embedded web extension in install manifest. (#2688)

This causes the embedded web extension to be parsed at startup, and
triggers a race condition in existing Firefox code during startup
on a clean profile between AddonManager and devtools code.

Removing this is not an issue for Screenshots because it delays startup
of the embedded web extension until "sessionstore-windows-restored" is
observed anyway.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1356394 for more info.

addon/install.rdf.template

@@ -14,7 +14,6 @@
     <em:type>2</em:type>
     <em:version>__VERSION__</em:version>
     <em:bootstrap>true</em:bootstrap>
-    <em:hasEmbeddedWebExtension>true</em:hasEmbeddedWebExtension>
     <em:homepageURL>https://pageshot.net/</em:homepageURL>
     <em:multiprocessCompatible>true</em:multiprocessCompatible>
   </Description>