docs: set language for Sphinx build

Sphinx 5.0.0 no longer accepts None for language. Remove it and Sphinx
will use English as the default and no longer emit a warning.

For details see sphinx-doc/sphinx#10474

Sphinx warnings cause our documentation build on GitHub Actions to fail.
Failures were observed for macOS builds which upgraded to Sphinx 5.0.0.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>

doc/conf.py

@@ -85,13 +85,6 @@ def fio_version():
 
 version, release = fio_version()
 
-# The language for content autogenerated by Sphinx. Refer to documentation
-# for a list of supported languages.
-#
-# This is also used if you do content translation via gettext catalogs.
-# Usually you set "language" from the command line for these cases.
-language = None
-
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used:
 #