[Driver] Remove Myriad.cpp

I am trying to clean up GCCInstallationDetector::init and noticed that
Myriad.cpp is the only toolchain using `ExtraTripleAliases`. This is a
little overhead, but I figured that Myriad.cpp is unused.
Its sanitizer runtime part was removed in 2021 by D104279. It seems time
to retire it.

Reviewed By: waltl

Differential Revision: https://reviews.llvm.org/D158706