godoc: use "IsPredeclared" of go/doc

In the go/doc, the same map as "predeclared" is defined.
Instead of the "predeclared" map use the "IsPredeclared" function
which is an accessor for the go/doc map.

Deleted unnecessary "predeclared".

Fixes golang/go#20357

Change-Id: I7fdf42cfeb4440fb490be9380f39035be88f9a5a
Reviewed-on: https://go-review.googlesource.com/43473
Reviewed-by: Kevin Burke <kev@inburke.com>