[Driver] Remove unneeded *-linux-gnu after D158183

As the comment added by a0772719
suggests, these `*Triples` lists should shrink over time.

https://reviews.llvm.org/D158183 allows *-unknown-linux-gnu to detect
*-linux-gnu. If we additionally allow x86_64-unknown-linux-gnu
-m32/-mx32 to detect x86_64-linux-gnu, we can mostly remove these
*-linux-gnu elements.