integration_test: drop "go build ./..."

This should be redundant with "go test ./...", and on pre-Go1.11 versions it
fails on directories that contain no non-test files.

Change-Id: I2c16cf3b26e67f3baa7732f0f7744aa547972e1a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/172237
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

integration_test.go

@@ -63,7 +63,6 @@ func Test(t *testing.T) {
 				})
 			}
 			workDir := filepath.Join(goPath, "src", modulePath)
-			runGo("Build", workDir, "go", "build", "./...")
 			runGo("TestNormal", workDir, "go", "test", "-race", "./...")
 			runGo("TestPureGo", workDir, "go", "test", "-race", "-tags", "purego", "./...")
 			runGo("TestReflect", workDir, "go", "test", "-race", "-tags", "protoreflect", "./...")