Refactor: remove function type check

[TomBinford]

This commit introduced the following check:

TeachLAFrontend/src/components/Output/Output.js

Lines 113 to 118 in 6f603af

	// check if getsrcdoc is a function
	if (!getSrcDoc && {}.toString.call(getSrcDoc) === '[object Function]') {
	return null;
	}

This was meant to verify that getSrcDoc() is safe to run, but

1. It should be !(getSrcDoc && ... ); as written the condition is never true.
2. We supply getSrcDoc so we'd get an exception and find the problem ourselves if getSrcDoc is not a function. The check isn't any more useful than an exception being thrown.