Drop the subsystem shorthand

[mikelangmayr]

• subsystem_prefix let start fei match hsfei_*. It compensated for a redundant prefix rather than removing it, and asked every instrument to decide whether its names have one worth hiding.
• A subsystem named hsfei is now typed hsfei. Instance names, all and command ordering are unchanged.
• Verified: hsfei and hscal resolve, fei raises TargetError, and start hspower hsfei keeps its order.