Fix incorrect flagging of nested environments. (mathjax/MathJax#3070)

[dpvc]

This PR fixes an incorrect error message about nested environments whenever the equation, displaymath, or math environments are used. There were two calls to parser.Push(begin) in

MathJax-src/ts/input/tex/base/BaseMethods.ts

Lines 1709 to 1722 in 49db078

	BaseMethods.Equation = function (
	parser: TexParser,
	begin: StackItem,
	numbered: boolean,
	display: boolean = true
	) {
	parser.configuration.mathItem.display = display;
	parser.stack.env.display = display;
	parser.Push(begin);
	ParseUtil.checkEqnEnv(parser);
	parser.Push(begin);
	return parser.itemFactory.create('equation', numbered).
	setProperty('name', begin.getName());
	};

but the first was supposed to have been removed:

7bb5fbd#diff-8b54d4a07ba56e1e6678245687ace65d468dd87cb4c8770a11b261c94d0231a0

Not sure how it got restored (maybe a bad merge or merge conflict resolution). In any case, the first one needs to be removed, as it forces the "Erroneous nesting" message to be produced whenever \begin{equation}...\end{equation} is used (and also affects the displaymath and math environments).

Resolve issue mathjax/MathJax#3070.

[zorkow]

lgtm.