Revert "godoc: use "IsPredeclared" of go/doc"

This reverts commit 144c6642b5d832d6c44a53dad6ee61665dd432ce.

Reason for revert: Breaks Go 1.7 and below.

See https://go-review.googlesource.com/c/43473/

Change-Id: I06ff6e9dccb842e29389af6beb67d6cdc217fb98
Reviewed-on: https://go-review.googlesource.com/43496
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>
1 file changed