Fix migration crash

[cewert]

I tracked this down to a bad line of code I wrote in migrations.bs. It's trying to use m.global.session.server.id but it will always be invalid at this point in the app when the function is called. We should just remove it instead of fixing it because what it's trying to do makes no sense anyway - the active server id is staying on the global "jellyfin" block and doesn't need to be migrated anywhere.

One bad thing about brighterscript is that the lines shown on error logs are for the transpiled files and not the source files. I forgot this and couldn't figure out how line 18 in migrations (from the error tracestack) was writing to the reg but when I checked build/staging/ the line number matched the right line of code: registry_write("serverId", m.global.session.server.id, savedUserId)

[1hitsong]

Fixes the 1.6.6 -> 2.0 start up crash I was having.