cmd/doc: don't bother checking validity of symbols at top level

cmd/doc: don't bother checking validity of symbols at top level

No need to check as pieces further down do so anyway:

	%  go doc '&&.%$^'
	doc: symbol && is not a type in package fmt installed in "fmt"
	exit status 1
	%

Removing this check allows 'go doc sort.interface' or 'go doc
types.type' to discover sort.Interface and go/types.Type.

Easily

Fixes #34656.

Change-Id: I84352e83dd7f91a232f45a44d1a52f019a1a9a06
Reviewed-on: https://go-review.googlesource.com/c/go/+/205778
Reviewed-by: Caleb Spare <cespare@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>