bootstrap.bash: remove exec wrappers

Without this change, building an Android toolchain fails:

$ CGO_ENABLED=1 GOARCH=arm64 GOOS=android ./bootstrap.bash
...
rmdir: failed to remove 'bin/go_android_arm64_exec': Not a directory

Change-Id: Ibc3b1e2fd24b73a63bd3020ce1e813f2b4496125
Reviewed-on: https://go-review.googlesource.com/c/go/+/170941


Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>