Do not install extensions twice in multidoc

[dvoytenko]

Partial for #33001.

For some reason, the multidoc is installing extensions twice. In reality they only install once since the extension service checks whether or not an extension has been installed. But there's no reason to do it twice. More importantly, the first call is more precise because it includes the correct version.

The first call is inside the mergeHead_:

amphtml/src/multidoc-manager.js

Lines 442 to 446 in 472974b

	this.extensions_.installExtensionForDoc(
	ampdoc,
	extensionId,
	urlParts.extensionVersion
	);

The second call is done right after the mergeHead_:

amphtml/src/multidoc-manager.js

Line 242 in 472974b

this.extensions_.installExtensionsInDoc(ampdoc, extensionIds);