[llvm] [RISCV] Remove IgnoreUnknown from RISCVISAInfo::parseArchString. (PR #97372)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> Isn't this functionality needed for things like llvm-objdump?..

Some digs into the history:
1.  We added this `IgnoreUnknown` in https://reviews.llvm.org/D139553 to ignore unknown extension when parsing ELF attrs.
2. @asb added `parseNormalizedArchString` in https://reviews.llvm.org/D144353 to do parsing that ignores unknown extensions and removed the only use of `IgnoreUnknown`.

So it seems we don't need `IgnoreUnknown` any more.

https://github.com/llvm/llvm-project/pull/97372