hostmaster-install references outdated file.

Aegir 1.1
drush hostmaster-install

During a manual installation the user is prompted with:

We are making the following assumptions:
 * you have read INSTALL.txt and prepared the platform accordingly
 * the FQDN of this machine is valid and resolves
 * you are executing this script as your "aegir" user

The INSTALL.txt file no longer exists. References to it should be removed.