[expr.const] Unmark introduction of "constant expression" as definition

[eisenwave]

Fixes #6589.

As explained in that issue, this definition is a duplicate. It's not actually a definition, it's just falsely marked \defnx. The actual definition can be found below:

draft/source/expressions.tex

Lines 7811 to 7815 in 78635c9

	A \defnadj{constant}{expression} is either
	a glvalue core constant expression that refers to
	an entity that is a permitted result of a constant expression (as defined below), or
	a prvalue core constant expression whose value
	satisfies the following constraints:

@frederick-vs-ja was concerned that removing definition formatting would drop the requirement that copy elision is not performed. However, that is not the case; https://eel.is/c++draft/class.copy.elision#1.sentence-6 explains that copy elision is not performed in constant expressions. The intro fluff in the first (duplicate) definition in [expr.const] is just repeating that.

[eisenwave]

@jensmaurer please have a look. This is bordering on a CWG issue, but I believe it's clearly just a misapplication of \defnx.

[jensmaurer]

Agreed; this is intro fluff and leftover from times when we didn't have the elaborate "core constant expression" / "constant expression" distinction.