fluke-45: drop serial port echo test, was disabled for years

fluke-45: drop serial port echo test, was disabled for years

The Fluke 45 driver's ECHO_TEST broke other people's SCPI device
identification and was disabled in 2018-11. Remove the dead code which
is impossible to resurrect. Its purpose was questionable to begin with.

Should users be affected, then the echo would break the identification
phase. The check cannot be done before identification without affecting
other connected devices, cannot be done after the identification either
when that very identification systematically fails. Should a user meet
this condition then applications' logs are available to see why *IDN?
fails, assuming that users are not aware of their serial port or device
configuration. The request's reflection in the place of a response gives
a strong hint.