SUMMARY: Remove entry for shell.md.

Author: flexibeast

src/SUMMARY.md

@@ -23,7 +23,6 @@
    - [rc files](./config/rc-files.md)
    - [Cron](./config/cron.md)
    - [Solid State Drives](./config/ssd.md)
-   - [Shell](./config/shell.md)
    - [Security](./config/security/index.md)
       - [hashboot](./config/security/hashboot.md)
       - [AppArmor](./config/security/apparmor.md)