go/build, go/doc: fix tautological conditions

These issues were found by the new vet's nilness check. The variables
were already checked against nil, so remove extra checks.

Change-Id: Ie252ccfcc755f3d06f691f354bf13d5a623fe17b
Reviewed-on: https://go-review.googlesource.com/c/148937
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: ainar-g

src/go/build/read_test.go

@@ -118,7 +118,7 @@ func testRead(t *testing.T, tests []readTest, read func(io.Reader) ([]byte, erro
 			}
 			continue
 		}
-		if err == nil && tt.err != "" {
+		if tt.err != "" {
 			t.Errorf("#%d: success, expected %q", i, tt.err)
 			continue
 		}

src/go/doc/reader.go

@@ -81,7 +81,7 @@ func (mset methodSet) add(m *Func) {
 		mset[m.Name] = m
 		return
 	}
-	if old != nil && m.Level == old.Level {
+	if m.Level == old.Level {
 		// conflict - mark it using a method with nil Decl
 		mset[m.Name] = &Func{
 			Name:  m.Name,