doc/scripts/hxtool.py: Strip trailing ':' from DEFHEADING/ARCHHEADING

In hxtool files, section headings defined with the DEFHEADING
and ARCHHEADING macros have a trailing ':'
  DEFHEADING(Standard options:)

This is for the benefit of the --help output. For consistency
with the rest of the rST documentation, strip any trailing ':'
when we construct headings with the Sphinx hxtool extension.
This makes the table of contents look neater.

This only affects generation of documentation from qemu-options.hx,
which we will start doing in a later commit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200228153619.9906-23-peter.maydell@linaro.org