Delete unused files (zarr-developers#1251)

* Delete build.cmd

This file was leftover from before numcodecs was split from zarr-python.
It was used to aid in compiling the Cython extensions on Windows.
However zarr-python pure Python. So there is not a need to keep this
file. It also doesn't appear to be used here. So drop it.

* Update release.rst

* Delete unused `.gitmodules` as well

This was for pointing to Blosc. Again not needed after the
zarr-python/numcodecs split.

* Drop `release.txt`

This is unused. Releases are handled through GitHub Actions. One should
look there if attempting to reproduce outside of that infrastructure.

* Drop unused `windows_conda_dev.txt`

* Generalize release entry

Author: jakirkham

.gitmodules

@@ -20,6 +20,9 @@ Maintenance
 * Simplify if/else statement.
   By :user:`Dimitri Papadopoulos Orfanos <DimitriPapadopoulos>` :issue:`1227`.
 
+* Delete unused files.
+  By :user:`John Kirkham <jakirkham>` :issue:`1251`.
+
 * Migrate to ``pyproject.toml`` and remove redundant infrastructure.
   By :user:`Saransh Chopra <Saransh-cpp>` :issue:`1158`.