ECMAddQch: drop trying to set IMPORTED on targets with installed config

This was done trying to resemble the properties of imported software
targets, though not read so far.
It missed though that IMPORTED is a read-only property officially,
setting it might have only worked by chance at some time.

CMake >= 3.29 now starts to warn about this, and will even fail over it
with CMP0160 being enabled.