models: remove Point.AddField method

AddField doesn't allow us to report errors, but we need to validate
some field names. Since we have to change the interface anyway,
we simply insist that all callers use NewPoint, rather than AddField.

Per advice from Jason Wilder, see

influxdata#5716 (comment)

Signed-off-by: Jon Seymour <jon@wildducktheories.com>

models/points.go

@@ -37,7 +37,6 @@ type Point interface {
 	SetTags(tags Tags)
 
 	Fields() Fields
-	AddField(name string, value interface{})
 
 	Time() time.Time
 	SetTime(t time.Time)
@@ -1215,14 +1214,6 @@ func (p *point) Fields() Fields {
 	return p.cachedFields
 }
 
-// AddField adds or replaces a field value for a point
-func (p *point) AddField(name string, value interface{}) {
-	fields := p.Fields()
-	fields[name] = value
-	p.fields = fields.MarshalBinary()
-	p.cachedFields = nil
-}
-
 // SetPrecision will round a time to the specified precision
 func (p *point) SetPrecision(precision string) {
 	switch precision {