*.hx: Remove all the STEXI/ETEXI blocks

We no longer generate texinfo from the hxtool input files,
so delete all the STEXI/ETEXI blocks.

This commit was created using the following Perl one-liner:
  perl -i -n -e '$suppress = 1,next if /^STEXI/;$suppress=0,next if /^ETEXI/; print if !$suppress;' *.hx

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>