depend-cleanup.sh: Simplify the logic, and clean bootstrap tools.

• There's no need to check if the file exists before grepping it; if it does not exist, grep will fail, which is what we want. Just redirect the error message to /dev/null.
• There's no need to split the .o and .pico cases; the bodies are identical, so combine the conditions. Use a glob to avoid a false negative if one exists but not the other.
• Also run depend-cleanup.sh on the bootstrap build tree. This unbreaks the build after 29c5f8bf9a01.

Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D38447