build(docs): use default language for Sphnix

Since Sphnix 5.0, setting language=None will trigger a warning
and this breaks the build. Since the default language is English,
remove this from conf.py.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: Idef97623fd4534ba6f81b2afcd7021f10434acd9