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

Recommit 435ea21c.
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.

Retain x86_64-linux-gnu for now to work around #93609.
(In addition, Debian /usr/bin/clang --version uses x86_64-pc-linux-gnu).
Retain i586-linux-gnu for now to work around #93502.