[clang] [flang] [lld] [Flang] Remove FortranDecimal library (PR #121997)

[Peter Klausler via llvm-commits]

klausler wrote:

can you accomplish your goals while leaving the binary/decimal package's sources and headers in their own directory?  they constitute a stand-alone package that would lose its identity if the files were to be scattered elsewhere.

https://github.com/llvm/llvm-project/pull/121997