Remove old PromQL artifacts

[brancz]

I was reading through PromQL parser/lexer code and realized there are old and unused artifacts in the lexer/parser from the old (pre 2.0) recording and alerting rule format.

For example:

prometheus/promql/lex.go

Lines 221 to 225 in dfdd2cb

	"alert": itemAlert,
	"if": itemIf,
	"for": itemFor,
	"labels": itemLabels,
	"annotations": itemAnnotations,

This should also simplify the lexer/parser, as only expressions will be left to parse.