do not leave repository mounted when starting yast (bsc#1196061)

[wfeldt]

Task

Port https://github.com/openSUSE/linuxrc/pull/286 to SLE15-SP3.

Original task

• https://bugzilla.suse.com/show_bug.cgi?id=1196061
• https://trello.com/c/bC5yVZas

linuxrc traditionally leaves the installation medium mounted when starting yast. This is not strictly necessary.

Do not do this, as this interferes with YaST's media handling and extension loading via extend.