[doc rm] removed file before moves for new docs

`util/tlgen/README.md` contains no useful contents and will be replaced
with the content of `doc/rm/crossbar_tool/index.md`.

To make tracking moved files easier.
This is being deleted in a separate commit.

Signed-off-by: Hugo McNally <hugo.mcnally@gmail.com>

util/tlgen/README.md

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-# TL-UL Crossbar RTL generator
-
-`tlgen.py` is a generation script for building TileLink-based RTL crossbar files.
-See the full [reference manual](../../doc/rm/crossbar_tool/index.md) for details on this utility script.