decoder/schema: Remove old (unused) logic related to legacy expressions

[radeksimko]

Context

As part of some recent PRs we introduced Constraint to schema.AttributeSchema:

hcl-lang/schema/attribute_schema.go

Lines 26 to 28 in 9c3a246

	// Constraint represents expression constraint e.g. what types of
	// expressions are expected for the attribute
	Constraint Constraint

This is now used as the primary way to describe attribute's RHS (expression) constraints.

This leaves us with the old/legacy way still in place which is now unused

hcl-lang/schema/attribute_schema.go

Lines 22 to 24 in 9c3a246

	// Expr represents expression constraints e.g. what types of
	// expressions are expected for the attribute
	Expr ExprConstraints

and also no longer covered by tests #251

Proposal

• schema: Remove Expr from AttributeSchema
• schema: Remove any logic related to Expr
• decoder: Remove any logic related to Expr