std: remove unsupported pipe module from VEXos pal
rust-lang/rust#146794 moved `pipe` implementations out of PAL and into a toplevel `sys` module. While most targets supporting libstd were updated, the PR did not remove the old module from the `vexos` PAL, causing builds to fail on the `armv7a-vex-v5` target.
<img width="2258" height="322" alt="image" src="https://github.com/user-attachments/assets/3bdb83f9-e577-4795-9d20-0ae4ab5d505c" />
This PR removes the old module path in the PAL and allows `vexos` targets to fall back to the `unsupported` implementation in sys/pipe.
f4d3c0a033