[experiment] remove the portable simd f16 patch

[folkertdev]

I believe this will fail but let's see what happens.

[folkertdev]

As expected

https://triage.rust-lang.org/gha-logs/rust-lang/rustc_codegen_cranelift/86638749132?pr=1675

  = note: C:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/15.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: D:\a\rustc_codegen_cranelift\rustc_codegen_cranelift\build\portable-simd_target\x86_64-pc-windows-gnu\debug\deps\libproptest-f4ca5451e39982af.rlib(proptest-f4ca5451e39982af.proptest.26d0c0f6a929e698-cgu.05.rcgu.o):proptest.26d0c0f6a929e698-cgu.05.o:(.rdata$.refptr+0x0): undefined reference to `floorf16'
          C:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/15.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: D:\a\rustc_codegen_cranelift\rustc_codegen_cranelift\build\portable-simd_target\x86_64-pc-windows-gnu\debug\deps\libproptest-f4ca5451e39982af.rlib(proptest-f4ca5451e39982af.proptest.26d0c0f6a929e698-cgu.05.rcgu.o):proptest.26d0c0f6a929e698-cgu.05.o:(.rdata$.refptr+0x0): undefined reference to `ceilf16'
          collect2.exe: error: ld returned 1 exit status

So that means we additionally need fallbacks for floorf16 and ceilf16. I'll look into that, seems straightforward.