Revert "build: fail nicely if somebody runs all.bash from a binary tarball package"

This reverts commit 3d86d45d.

Reason for revert: Fixing a different way. See #20008.

Change-Id: I6f4382ffdfff83a4504b013ba37100319b2c1aad
Reviewed-on: https://go-review.googlesource.com/42138Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/all.bash

@@ -8,10 +8,6 @@ if [ ! -f make.bash ]; then
 	echo 'all.bash must be run from $GOROOT/src' 1>&2
 	exit 1
 fi
-if [ ! -f cmd/api/run.go ]; then
-	echo 'all.bash cannot be run from a binary download package' 1>&2
-	exit 1
-fi
 OLDPATH="$PATH"
 . ./make.bash "$@" --no-banner
 bash run.bash --no-rebuild