docs: drop %.png: %.fig rule

convert bin is part of ImageMagick package and uses uniconvertor to
create png file from fig files.

Unfortunately uniconvertor is python2 only and not available in most
recent distributions which makes the convert command fail with:

sh: uniconvertor: command not found
/usr/bin/mv: cannot stat '/tmp/magick-1397138DRT8Pzx4Qmoc.svg': No such file or directory
convert: delegate failed `'uniconvertor' '%i' '%o.svg'; /usr/bin/mv '%o.svg' '%o'' @ error/delegate.c/InvokeDelegate/1958.
convert: unable to open file `/tmp/magick-1397138S8ARueJXLXkc': No such file or directory @ error/constitute.c/ReadImage/605.
convert: no images defined `docs/migration-managed-direct.png' @ error/convert.c/ConvertImageCommand/3226.

It looks like that there are plans to somehow port uniconvertor into
python3 but as part of different project color-picker but the job is
far from complete.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

docs/Makefile.am

@@ -406,9 +406,6 @@ hvsupport.html.in: $(top_srcdir)/scripts/hvsupport.py $(api_DATA) \
 	$(AM_V_GEN)$(RUNUTF8) $(PYTHON) $(top_srcdir)/scripts/hvsupport.py \
 		$(top_srcdir) $(top_builddir) > $@ || { rm $@ && exit 1; }
 
-%.png: %.fig
-	convert -rotate 90 $< $@
-
 manpages/%.html.in: manpages/%.rst
 	$(AM_V_GEN)$(MKDIR_P) `dirname $@` && \
 	 grep -v '^:Manual ' < $< | \